The other guides detail how to play the game and enemy attack patterns.
But I want to list some of the subtle features of the enemies not already mentioned in the other guides (as such I don't have the basics in this guide).

I'll use the enemy names that BS|Couchfighter uses in their excellent guide!

Only ever one on screen at a time

• Never more than two on screen at a time
  
• Their attack timer doesn't seem to get interrupted when they take a hit... so be quick with follow up hits!
  
• occasionally you might get surrounded with test subjects on either side, sometimes it'll be so close you'll need to use this safe attack pattern: forward, back, back, forward, forward, back

• Never more than two on screen at a time
  
• When hit they are knocked out of range, unless you attack quickly (highly recommended)

• Their first teleport is half a screen away in a random direction
  
• Their second teleport is approximately a quarter of a screen away in the same direction as the first teleport
  
• The Red Ninja suffers a long interrupt when hit... if hit a second time while interrupted they will start moving again (following the standard teleport pattern of course)
  
• When the Red Ninja teleports on top of other enemies you should treat that as a new single enemy (more or less) and have an attack pattern in mind

Red Ninja (Front, Back) plus:

• Nazi Soldier = Front, Front, Back
  
• Nazi Commander = Front, Front, Front, Back
  
• Kung Fu DNA Test Subject (Red Ninja Slightly in front) = Front, Back, Front, Back, Front
  
• Kung Fu DNA Test Subject (Red Ninja Slightly behind) = Front, Back, Front, Front, Back
  
• Kung Fu DNA Test Subject (can't tell) = Front, Front, Back, Back, Front (do this fast! the Test Subjects attack doesn't get interrupted in the same manner that other enemies do)
  
• Kung Führer Robot = Front, Front, Front, Front, Front, Back (the Back can be done earlier if you feel comfortable... Or not at all, as normally you don't need to instantly respond to the ninja)

Red Ninja (Front, i.e. last phase or if you left them half way through the second phase) plus:

• Nazi Soldier = Front, Front
  
• Nazi Commander = Front, Front, Front
  
• Kung Fu DNA Test Subject = Front, Front, Back, Front
  
• Kung Führer Robot = Front, Front, Front, Front, Front
  
• Red Ninja = Impossible

• Taking damage will push enemies away from you and also slow their speed down (until you make an attack)
  
• Your attacks queue so you can issue multiple attacks and they will happen in sequence, unless you miss or get hit

• Focus on queuing up attacks for the enemies, don't focus on the attacks landing (you're Kung Fury, once you throw a punch it will land! except when you messed up)
  
• Turn the sound off, you don't need to hear the hits landing (on the robots for example), but you will benefit from hearing how many times you clicked the attack button!
  
• Practice and take breaks! (really!)
  
• Sometimes it's best to not try and kill the Robots all in one go if you aren't sure all 4 hits will land before they leave range
  
• The Red Ninja will never be in a position to immediately hit you after you land a hit on him, he will either be in a long interrupt, or they will have teleported away (or is dead ;)... with that in mind sometimes it's safest to hit him just the one time.

It took me ages to break 250k, now 250k is a failed warm up round (my high is 31 million currently)

Here's a 25 million run, showing the concepts in action: