Im currently on wave 70, and have 100+ guards, I can't even see the enemies spawn cause my guards insta kill them... I think you should maybe mut a max on the guards or atleast increase their cost so it dosnt become so easy :P
Nice game, but the last boss took a while to kill, i recommend to kill the last boss change ur sword to blood blade and just let the mage die if u dont have any more ethers... :P then u just heal all the time on attack and drain and when he uses that soul ball attack u use defend to survive that one aswell :) Good Luck ^^