Stay with your pistol until wave 5. Save your money.

Then get two sentry guns. Place them around the air stick in both corners. Then get the riout squad or the regular squad. They should make a human body guard. Let the sentry gun do all the work.

When the helicopters come use predator missiles to take them down. Then your squad will probably be dead so buy another one.

When juggernuats come just let the sentrys do the work. Also you might want to flash bang him. He'll destroy your squad.

Oh yeah, if all your squad die just keep buying them. Once you get to round 15 buy a predator missile. Take the first one out, then take the other one out.

If your sentry goes down buy another one.

So pretty much just keep going like that until you die. You can use this solo or in multiplayer.