That gets pretty darn hard. More so in the sense that, the enemies rush you and a lot of them all at once. Add to that they take lots of bullets to die, then it's pretty difficult.
Earlier today I got to wave 45 on solo underground which gave me the rank of 13 on the leaderboards XD. (that's on Ps3 and that was my 2nd try on the map)
I haven't played much really but I have played a couple of games co-op and got 30 on Underground and a also a couple of games solo and managed to get 26 on Bakaara. It gets nasty when you have Juggernaut & Chopper & bad guys all at the same time on like wave 6
