Underground Survival Map:
I think the strategy for this map is pretty obvious. There is a tunnel that is fairly easy to defend, if one person stands at position 1 and the other at position 2.
For all regular waves (non juggernaut waves) one person has to sit at position 1 for the first few seconds of the wave in order for all of the enemies to go to the position 2 opening. If both sit at position 2 then many enemies will funnel down the stairs at position 1.
So at the beginning of the waves one person sit at position 1 for 10-15 seconds, then make sure there are claymores at position 1 and go down to help the guy at position 2 fight off the enemies.
I put a sentry gun facing up the tunnel as well. This is good for getting into the mid-20's easily. I played with a guy from Australia with lag that nearly made the game unplayable, and we still got to wave 26. I played solo and got to wave 29 the first try.
Underground is a Tier 1 map.
Helo waves: 6, 23+
Juggernaut waves: 10, 15, 20, 26, 30, 34