I bought two heros, the second last and the dragon. Acturally the second last is much better than the dragon, cuz it's a GROUND unit, which can block & fight in front of you gate, but the dragon is a FLYING unit and mobs just go passed it. One mob would fail you in those challenge modes. :D
met a bug, my troops just stop running forward after clearing a middle wave of enemies. i summon that skill % bonus skill and my troops killed the wave, then they just wait for the % bonus timer wear down ,then they just do nothing at all
i'll look into it soon as i can