Cave Of Trail
The Cave of Trails are the hardest places in game. once you go to cave of trail 1 example elite units will span, anda heck lot of them. and at some point a "Titan" will spawn (you hav might seen it at last lvl) anyway, if you beat the cave of trail you get the Titan card and can make it as you're own unit! but only if you are a patient person i would use it.
A good tip to beat cave of trails:
at cave of trail 1 -4 the thing you have to do, is getting the "pheonix spell" and use it when they use what i call "rewenge wave" thas when alot of enemys come out of nowhere, happends when castle is at 75, 50 and 25%...... also when the rewenge wave come then you might wanna pull you're units back to castle and then charge. some times you only need 2 do this 1 time. well hope that elps =)