to beat Survival you have to beat 100 enemy. Once you get past 80 or so you start to battle the Alt Costumes of the characters. I believe you can unlock the unlockables on easy it that helps. I unlocked Karai's Alt Costume on easy. Hope this helps.
I posted a way to do it in the cheats section but they didn't accept it. They'd rather accept codes that don't exist.
Here's what I put though
This works with any character. Stand in front of where the enemies are spawning in the dojo and face the wall. The second they spawn use the wall grab on them. This will KO most of them instantly. As it progresses you may need to hit them again to KO them. Just make sure you block so they don't stun when they get up. Be careful with Night watcher and Donatello, their reach will cause problems. Make sure to stay blocking and dodging if they manage to escape or more then one spawns. Keep doing this and you should win in no time.