Huzzah, first thread!
Anyway, I've tried upgrading different units, changing the maze, but to no avail, the knight and mage are too hard.
I hope that this part is beatable, and not just something the designers put to end the demo.

It is beatable you just suck too much !

quote Gorchio

It is beatable you just suck too much !

*bleep* answer.... I have the full version and the third stage seems to be hard for me too, try making stronger monsters, poke the bones of fallen heros to create skelletons but there is also a better way of using them, when a hero dies he leaves bones where he died and rocks that spawn ghosts around them, destroy the blocks that spawn ghosts and when the bones turn red poke them with the pickaxe, you will get a rly strong skeleton, however he will disapear after a while so dont make it when no hero is around.

ok i found a way to beat the 3rd/4th lvl without any problem.

1st hero: let the slimemosses kill him or make an omnom close to him though kill it when hes done so they wont spread.
2nd hero: when hes close to the bones of the first hero poke the bones, the skely should deal with him, i prefer to poke them when hes close so mana stacks in same place.
3rd heros: Now dig the mana making it go to the bones and make em red or even more (i think it makes him more powerfull with more mana even above the red level). the red skely should kill 1 o maybe both heros but by this stage u should have some lizardman on the back huh?
4th and above: now u should have a nice army of lizardman on the back since they werent interupted for 4 levels, try making atleast 8-10 demons on the very end of the dungeon and have em ready for wave 5.

And i cant help u more, so far this is the easy way to get to lvl 5 and above but the rest is up to u , however i found this way rly good coz its easy and it savs dig power for upgrades , btw after 1st lvl only buy 1 slime moss upgrade, at lvl 2 dont use dig power then at lvl 3 u will have alot and then or on lvl 4 u should upgrade lizardman, when you dont use the digpower it will go up by like 35% by the next round so dont use upgrades too early too , for example if in stage 2 u have 500 dig power left and u know that the red skely + lets say 2 lizardman to be sure will deal with the heros then dont use it on upgrading lizardman , it will make atleast 700 dig power in your next score.