Is there any rhyme or reason to when diamonds are dropped? I've killed every human on fire for 100 levels without seeing a single diamond drop, and I've gotten 2 in one set of humans. (generally the 100 levels without)
Okay gang, here's the shortcut to beating everything in the game. Forget the challenges for the moment, they're not important for anything anyways including the achievements. Just do main campaign for a while, upgrade yourself first (I recommend damage to hurry things along), then after about level 3 of everything, upgrade the rogue, then the barbarian, than the archer. Rinse and repeat until second to last stage, farm that for a few times until all 4 of them have at least level 3 upgrades for everything. Then do all the challenges (for the sudden death one just max the hero's speed upgrade and run in a circle around the outside edges of the stage and use the spin move/wave move), make sure you max all the upgrades before the final boss, use max runes, roll right before he attacks and make sure to kill minions first. With the healing rune you should be able to outlast him. When he gets down to the last 10th of his health bar, ignore the minions and attack him like crazy.
So... people that are complaining about the sudden death challenges... just upgrade speed, and buy the damage runes to max. If you go in a circle the enemies will group up in the middle and you can just use the spin move, dodge away rinse and repeat.