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This is pretty much exactly the same as defending, but retreating usually means you are in a battle that has gone wrong and you want to get out quick.
The best move is usually to dive (by which I mean jump like a loony in the general direction of) behind a box. Any sort of cover where the enemy can't shoot at you. Don't bother reloading, throw a flash bang. This will blind the enemy, and it doesn't really matter if you get yourself, just remember where the exit is before you do. Then run very fast in that direction, while jumping like a nut ball and shooting randomly in the direction of the enemy.
Hopefully the enemy will be injured and blind, these are not the best conditions in which to jump into the line of fire of a mad man, so they will usually wait a moment - giving you enough time to leave the area. Don't stop then, keep going, run and run and run. Reloading all your guns on the way, and figuring out your next course of action. More often than not it's go the other way. Refer to the defending section if it's got especially bad on what you should do next.