If you want to kill xenomorphs to rank up, the first thing you want to do is to go in multi-player and then click on player match and then you will click on create game.
Now after you have done that you want to change the mode to survivor and then you can start the game. It is like single player survivor except you rank up by it. You rank up depending on how high of a score you have, although it gets harder to rank up the more ranks that you go up to.
One thing that might be just me but if you equip a character that isn't the default one the xenomorphs would immediately attack you instead of just waiting until you get close to them.

There is some point in the ranks where you would get higher ranks with campaigns but the "points" would go down if you would use health shards as a predator or if you use health stimulants as a marine. If you harvest someone as a xenomorph that would count as a positive point. I may be wrong about this but I am pretty sure that as a marine it counts when you kill aliens, though it doesn't count if you destroy an ovomorph whether it is open or closed, the same applies with the facehuggers because they can be killed in one shot.

If you found another way to rank up without having to sit in quick match for a long time you can comment it if you want, though you don't have to.



By the way ranks are what you use to unlock skins like for an alien it would be ones like nethead, praetorian, warrior, warrior dome, etcetera.
By the way the ways that are listed do not give you any experience (EXP).