this is a guide i made and researched by game play:
first the area i researched is the dungeon, large area.the reason i chose this one is it has cover, multiple enemy respond points, and areas with none.here is a quick drawing of the area i made. http://img20.imageshack.us/img20/1035/thingthing3mapdl.png

1)area 1 is the best to have few enemies and still recharge your health.
2)area 2 is a death zone if it is hordes of enemies but still has ammo.
3)area 3 is good for executions(stun an enemy with a shot then hit F and to execute them hit F again(earn 30 points)).
4)area 4 is good for action and fast kills but get surrounded easily when it gets difficult.
5)area 5 has an advantage, you can have your back facing the wall and shoot up at them and not get hit.
6)area 6 is a heaven for healing, it has few enemies coming at you and a wall to hide against.it also has ammo.
7)area 7 is the all around best place to stay. you have hordes coming at you at your own expense(if they stop coming from above simply jump up till you see enemies coming, then lure them to drop down.).
8)area 8 is best for luring people to area 7.
9)area 9 is hard to fight in because you become surrounded easily, but has ammo in it.

glitches and tricks:
*when you are close to an enemy, stand on the same spot as him and he nor you can kill each other(not including the pipe).
*when performing a execution on an enemy, you become invincible while doing it(including gas, bombs, grenades, everything).
*my personal favorite gun is the dual handguns because up close you can hide in there gun and still shoot them.
*holding down lowers your enemies ability to shoot you and you stay less see able.
*when you use the pipe in execution, you can take off there head and keep it on your pipe(will break after four hits). but if you switch to your guns and execute some one, it will not break till you use the four hits(the head increases the pipes damage on the enemy.

thanks for reading this, post your tips and comments.