Windward or Lost platoon are rather good.. Lost platoon is nice and open so you dont find yourself cornered at all but it lacks general cover.
Windward however is extremely easy to defend. Me and my friends had 37 lives to spare at the end of it. so we had our own little slayer match afterwards >:D
Play on Lost Platoon and control Choppers and Laser. That's all you really need to do. One laser guy gets laser every round and ONLY uses it on Wraiths. Stockpile spare choppers in the base whenever possible. I got 2,000,000+ points doing this. :)
On alpha site, you can move the crates so they block the stairs it slows down the incoming enemies to a trickle because they have to jump over it. That's pretty much how we got the achievement, we stayed back and as soon as they jumped on the crate we would pop um in the head. For brutes just use the needlers that are dropped, for drones use the SMG, for chieftains all I can say is run like you've never fucking run before, then shoot, if your really good, you can assassinate them. If you think of the maps line of symmetry, the rocket spawn is over a bench near then enemy spawn. You can grab it when each round ends, same for needlers and other ammo if you run out of pistol. That's my guide to alpha site.
Edit: On bonus rounds, there are two platforms above the middle of the map, above that are four windows, grunts will drop out of the windows onto the platform onto the floor. If you have every guy with a pistol each aiming at a window, you should be able to get bonus lives everytime.
6:19 AM <Something.> OH GOD WHAT ARE YOU PEOPLE TALKING ABOUT
6:11 AM <oh knarly> I like big butts and I cannot lie..
6:09 AM <Xylom> I like it sweet and sour.
6:09 AM <oh knarly> With a burger.
6:09 AM <oh knarly> I like it saucy.
5:58 AM <Chronik> I like it wet.
5:53 AM <buddhacrane> I like it sticky
5:42 AM <ShaddoBlade> I like it rough.
Alpha site. As long as you follow the same stay back behind the stairs tactics, it's very easy. I got to the 4th set with 28 lives, and then we started committing suicide after getting the achievement. I also recommend you DON'T use the rocket launcher... I found personally that on the higher waves they were useless and really only limited you because you had something you don't want to use with very limited ammo. On the higher waves when all the skulls are on, carbine/magnum and plasma pistol combo is your best friend. Plasma pistol takes out shields with one overcharge and it'll only take one shot for a headshot instant kill. Oh, and it'll help you with the 'my clothes' achievement. Yeah... other than that the other advice I can give you is stay away from middle open area as possible, only go there for ammo, and keep your VISR on as much as you can. I found VISR to be really helpful on alpha site. For the bonus rounds, as xylom said aim for the open windows at the top where they flood in mass out of. I've gotten a killionaire from it, which gave us an extra 4 bonus lives.
Alpha site is the best I think, I haven't gotten the achivement yet, but my friends and I have gotten the farthest on that map than any other. If you just stay back and not tread out into the middle it works fine, we learnt that the hard way.