I found the best way to get to high level 20's in dome is, when you first start run out the doorway to your immediate right and go up the ladder on the dome, then lay down right in front of the ladder, almost all the enemies will come up the ladder and you can get all head shots and kill streak bonuses, keep in mind you will get an occasional enemy from your right side that climbed up the other ladder, just look over and pick them off, after the second wave you should have between $7000 and $10, 000, run back into the brick building and buy 2 sentry guns and place them outside between the brick building and the dome all the way against the cargo containers that make a wall, 1 on each side of the forklift facing up the hill towards the enemies path, also picking up all the shotguns and getting back up the ladder, after wave 4 is done you should have about $10, 000 to $13, 000, go in the dome and buy a light machine gun and trade your shotgun for a AR, also go fill your grenades, now your pretty well armed, on level 6, 15, 24, you will have helicopters, just grab your sentries and run into the brick building placing each one in the back corners, and pick off all the enemies and save the choppers for last, if you stand in the far back corner there is a pillar that you can stand behind that you can move over and shoot the birds thru the hole in the roof and then duck back behind when they shoot back, after about 1 clip in the LMG timed right the birds should be down, then just put your sentries back outside and get back up the ladder and continue shooting the fish in the barrel, also while laying down you can shoot down thru the floor your on and pick them off before or coming up the ladder, on wave 9, 12, 18, 21, 27 you will have Juggernauts, just crouch on the edge of your platform and when the first one hits the ground from the bird above hit him with a stun grenade and empty a clip from your LMG and he's done then just stun the others and keep your distance and pick them off, NOTE: once you hit 24 the games throws everything at you LOTS of enemies and it gets tough, you will end up jumping out of your nest and running for cover else where, the farthest I've made it is to the last jugger on level 27, I was shooting the second one and the third killed me from behind.