I havent tried legend yet but I made it through the 50 waves 4-5 times on normal using matchmaking...the biggest thing I learned was dont waste scrap on the tripwires, save for the decoys. That has helped big time with the tormentors & ogres...I have reached a point several times were there are no more enemies but nothing is happening only to realize an unstoppable ogre is in another part of the map going off on our decoy. I usually try to get the tormentor to follow me away from the ADU towards a decoy then when he starts attacking it dip out & circle back for the counter. Its nothing Earth shattering but making that change made getting through all the waves significantly easier for me.