The first challenge was just warmong on Magma Mines. The second was Warmonger on Magma Mines, killing the foreman first and physical resist boss second. Obviously half-dragon makes killing the physical-resist boss easy, but you're basically disqualified from killing the foreman. Still possible with chaos avatar or some very weird stone heart antics, but good luck doing it.
What other challenges might be nice? Oh, I know: defeat the boss of Grimm's Grotto as a level 1 character. I've done it as a level 2, though you will definitely need to scum for a low-HP boss.