Know your roles:
Needless to say, knowing the capabilities and limitations of each role in your squad is important

Rifleman:
Every member of your team starts as a rifleman. There isn't much to say about him, you should have him figured out within seconds of starting a match. One thing worth mentioning is that he's more capable than you might think. A single rifleman can hold off hoards of aliens for an extended amount of time. Replace him when fighting Brutes or attacking a Hive with Reflect.

Fusion Shotgun:
Think of him as "Rifleman+". Range similar, higher damage and does damage in a cone shape.

Medic:
Besides the obvious, the medic has a slow firing grenade launcher, great for carving out chunks of grubs. Usually good to have early in the game, especially if you don't have bunkers.

Sniper:
Slow firing, über long range. Doesn't do much against groups, but is invaluable when attacking a base head on. The sniper will clear out tower from a safe distance while your other marines move up. Also a great counter agianst Clones! Remember that the sniper, unless otherwise ordered, will take priority attacking any towers within range, so you don't have to tell him to individually attack each tower.

Minigun:
Effectively turns your marine into a mobile turret. He does pretty much the same amount of damage and slices into enemy lines just as easily. Can take relatively enormous amounts of punishment! Stop using him as soon as Brutes appear.

Flamethrower:
Can hold off thousands of aliens (all types to boot!) single handedly. The range is short but he makes up for with quick movement. A must have whenever alien hives start to Reflect and/or when Brutes join the battlefield. Also great for building since they can get back into the fight much quicker.

Scount:
Very fast and very invisible. He's strong against all targets, and is great for assaulting hives. Just remember to keep your ammo up, if you run out, he will lose invisibility. Another great counter for Clones.

Know your enemy:
Knowing what you're looking at when staring down the alien hoard will be invaluable. This will help you counter. I won't list every last type here, because most of them aren't tricky at all. For example the early game "grubs"

Brutes:
These beetle-looking bugs are slow, but have two notable (and pain-in-the-ass) features: They take a decent amount of damage and they Reflect. Counter with Flamethrowers and/or Medics. Turrets will chew themselves up fighting Brutes, so don't count on them. Basically, avoid bullets, embrace flames and explosives.

Towers:
These purple pains can be the bane of your existance depending on Mutation combinations. Not many tricks with the tower, just try to avoid going toe-to-toe with them with anything other than Snipers (which will 1-shot them from afar) and Miniguns. Early in the game, simply dedicate your ammo to using grenades to 1-shot them.

Clones:
Probably the most troublesome enemy. Clones of your men appear under different circumstances. Namely, as Mimic "eggs" whenever a hive is destroyed after the "Mimicry" mutation, or when the aliens launch a "clone attack". Be mindful of the indicator that shows which hive they're coming from and where they're going (looks like a bunch of pink bubbles on the battlefield). The clones will appear as the same classes as your squad at the moment the message "Clone Attack Iminent" appears. So changing classes after the warning won't do you any good. Just be mindful of what your squad consists of. Try to place a sniper (more than one, preferrably) along their axis of advance. Snipers and Scouts tend to be decent counters to Clones.

Know your hotkeys:

A - Attack and Move at the same time
D - Recycle
R - Tech Menu
F - Weapon Menu
V - Build Menu
TAB - Select All
1-0 - Select Group 1-10
Ctrl+1-0 - Bind Group 1-10
SHIFT - Queue up orders/waypoints
Memorize your hotkey combos! For example, sometimes a turret needs to be put up FAST in the heat of battle, so you for that you quickly hit "VW". Or you need to switch a rifleman to sniper quicky... "FZ"

Know your mutations:
There is currently no official list or guide covering the mutations in the game that I could find, so I'll list what I've run across thus far. Please message me if you know anything in addition to what's mentioned below.
Rust:
Let's alien poison attacks hurt anything metal a whole lot more. Think turrets and bunkers. Only counter is to keep the aliens at bay, which you should be doing anyway. Just be mindful.
Virulence:
Medics and bunkers no longer heal poisoned Marines. Time to go on the offensive, use those snipers to take out the towers before they can poison you, be prepared to switch to Medics when they're no longer poisoned.
Mimicry:
Spawns 3 clone "eggs" when a hive is destroyed. These will hatch after only a few seconds so act fast. Make sure you have at least 6 ammo when the hive explodes so you can grenade the eggs as a counter. Sometimes 1 grenade does the trick, although I find not always. If clones do spawn, counter with snipers or even better, scouts!
Hive Spit:
Hive essentially acts as a tower. Again be mindful and plan for that incoming damage. You should be OK with at least 1 Medic in the squad.
Brutes:
Hives start spawning Brute aliens. They Reflect, so ditch any weapons that fire bullets when fighting them!
Tower Spawn:
Releases a creature that travels to a specific location and "deploys" into a cluster of several towers and spawn sacs. You get roughly a 10 second warning when a tower spawn attack starts. Be mindful of the indicator. If possible, use Snipers or grenades to stop them. If they do end up deploying, use a helicopter strike (preferrably 2) to get rid of it quick.
Neurotoxins:
This mutation causes your squad to slow down significantly when hit by towers. Not much you can do to counter since you should be avoiding being hit by towers in any case. Just be mindful when approaching a hive and don't forget that snipers will 1-shot towers from safety.
Bombardment:
Causes towers to increase signficantly in range. Again, be mindful and take solice in the fact that snipers are still effective even after this mutation.
Impact:
Pushes your squad members away when hit by a tower, effectively giving towers the ability prevent your men from closing in. This is especially dangerous in conjunction with bombardment and neurotoxins. Again, snipers, or even Miniguns. Double or triple Helicopter strikes with clear them out as well.
Consumption:
Hives can consume towers to gain back health.
Barrier:
Alien hives create barriers to protect themselves. Choppers do a great job of clearing these out and Miniguns go through them like a hot knife through butter. If neither are an option, remember you don't have to destroy them, you can always go around!
Hive Shell:
Creates a shell around the hive (who woulda thunk?) when its health hits about 1/3. Bullets are reflected. Flamethrowers are great for this.
Counterattack:
Everytime you take a point, every active hive will spew forth a greater number of enemies which travel at a significantly fast speed. The effect is temporary, but repeats for every point captured.
Bulwarks:
A random hive deploys a Bulwark creature. When this creature attacks or gets killed, it turns into a cluster of towers surrounding a hardened core. Until this core is destroyed, the towers will keep respawning and FAST. Snipers do little to no damage against the core. As with Clone Attack and Tower Spawn Attack, watch for the indicator. Your best bet is to kill the towers with snipers then move in quick with flamethrowers or miniguns. Helicopter strikes are a good alternative.
Regeneration:
Hives and their surrounding towers regenerate health quickly. Plan your assaults accordingly, make sure you're able to keep up sustained firepower, because the second you let up the hive will be on its way to full health.
Necrophage:
When a squad member is killed, an egg is left in his place which hatches into a clone. Be mindful and have grenades on hand to pop the eggs before they hatch.
Guardians:
Towers are replaced with Guardians.These are aggressive aliens armed with shotgun type weapons that will actually attack you, not just defend the hive. When hurt, they will retreat back to their "eggs" to heal. This is when they're most vulnerable! Grenade or Chopper Strike accordingly! Also remember that 2 Snipers will kill 1 guardian instantly, so try sniping them and pulling back while the rest of your team moves in. Sometimes you just have to outgun them with Miniguns and airstrikes.
Guardian Nest:
A creature is deployed that spawns a guardian nest, counter as you would guardians if it deploys. Ideally, Sniping of grenading the creature will prevent any guardians from spawning whatsoever.
Calcification:
For a short time after everytime you capture a point, dead aliens leave behind a temporary barrier. Counter with non-bullet type weapons.
Hive Hardening:
Hives deploy a shield if struck by explosives. Bullets and flamethrows my friends! Helicopter Strikes are far less effective.
Carapace:
Towers grow a hardened shell around them. Remember that all barriers in Infested Planet are suspectable to explosives. That means medics, choppers and grenades!
Spitter Spawn:
Spitters are basically grubs with mini tower-like attacks. They come in large numbers but are fragile. Spitter spawn will randomly spawn spitters that attack your positions. Counter with Medics and Miniguns. Riflemen will work as well as turrets for defense.
Spitter Guard:
Spitter will defend hives as you attack them, see above for counter.
Sharp Spikes:
Poison spikes will deal part of their damaged instantly, as opposed to over time. Have those Medics on hand!
Burrowing Hives:
Hives occasionally will appear in your territory! Yep! Ideally, a triple chopper strike will stop this mutation dead in it's tracks. Either way, get to it quick! Set up turrets or split your team to hold the line, while everyone else attacks the hive. The longer it stays, the more trouble it will be.
Mimic guard:
Sigh... ahh the Mimic guard. Clones will now defend enemy positions. Basically, Clone "eggs" will appear in enemy territory, eventually hatching into a Clone. These eggs stick around for a while, use the opportunity to grenade or Snipe them! If possible, send scout(s) on egg sweeping runs when needed.
Infestation:
Dead human structures become infested. You should be avoiding your structures be destroyed anyway, so be mindful when leaving turrets by themselves to defend large sectiosn of your territory. A double or triple airstrike should save you sometime and soften the target before moving in.

Here are some simple and basic tactics to help you get a good foundation.

Tower Assassination:
When approaching a hive, have at least one squad member become a sniper (hotkeys FZ). When the hive is longer surrounded by towers, have the sniper switch over to medic (FW), when healing is no longer needed, swtich him to flamethrower (FC). This way, hive defenses are destroyed, the rest of your men that said defenses did hurt will recover, and whatever is left is quickly destroyed by the flamethrower. Repeat as needed.










Turret Leapfrog:
This tactic helps your create front lines and helps you manage your attention. Place turrets along your axis of advance. As you move up, recycle them and move them up ahead of you, effectively moving the front lines forwards and pushing the swarm back. Be mindful of alleys leading around your turrets, placing a rifleman (or flamethrower) in these alleys is a good way tp patch any holes in your lines. You can then focus on assaulting hives moving forward. Try it with Siege Cannon(s)!



Air Superiority:
This is simply the act of building a Helicopter Strike and Extra Ammunition. When your Strike cools down to 0 and you have full ammo you can launch THREE simultaneous Helicopter Strikes, this is great for softening up a hive before your men roll in. Just remember you will be virtually grenadeless going in.

Don't give up!
At some point, no matter what you do, eventually your lines will break. Whether it's a Counterattack or a Clone Attack or something else, something will break your defenses. Your lines will fall. KEEP YOUR COOL! All of the BP you've earned up to that point is still yours to keep. Even if you get pushed back to one point, you'll have plenty of resources to fight your way out. Just keep a cool head and push back one hive at a time.

Don't underestimate your men!
Remember that even a rifleman can hold back hundreds of aliens, even coming from multiple directions. Don't be afraid to split up your squad in 2 or even 3 teams (you'll often have to). They might look overwhelmed, but even as aliens get close your Marines will use melee attacks.

Think containment!
Assess the map when you begin. Often times, there will be hives in every direction. Meaning you're surrounded and will have enemies coming in from all directions. There are no "front lines", that means YOU need to create them. For example, if you start close to the northern side of the map, take the hives north of you with 3-4 men while leaving 1 (or a turret) to guard the south flank. Once every hive north of you is taken, you now have a front line to the south. You have no enemies "behind" you. No matter the layout of the map, think along the lines of this containment strategy. Hold them off from one direction, focus your attack in another until that direction is completely under your control. This way your attention isn't split in multiple directions.










Evolve your squad!
Remember, you can switch the roles of your men on the fly. Everything you spend your hard earned BP on can be refunded, instantly. Towers giving you trouble? Get a couple of snipers. When they're done, make them medics to heal up your squad. Squad is fully healed? Flamethrower or Minigun. So aliens got past your lines and are making a b-line for your vital buildings or need to build something far away? Turn one of your men into a Flamethrower or Scout to hurry over, then switch him back when he's done. I often find myself changing the roles of my men very often, possibly every several seconds, especially when assaulting a hive.