I really wish I'd come in here before spending lots of time beating Knightfall 2 without buying anything. On the bright side, now I know it can be done and I have some neat tricks for it.
Also, for the mirror image boss, I used the Armageddon spell and it killed all 3 right away. I also used the Armageddon spell near the end of the final boss, to get rid of the 5 orbs and the thief that were on the screen.
Also also, if you can line up 4 or 5 dark knights in a row and drill them all, the XP you get is insane. I was able to line up 4 of them, and got 20,000XP for it. I finished my game without buying anything and without dawdling at level 15 with something like 239K XP.