Stumblebum can be any level. All it aids at leveling up is the aim debuff. Which is really only useful to support your evade-stacking ninja-buddy like I do. But it's a very useful catch as it ignores evade and block.
Death Bound's curse Debuff lowers base evade among the other stats lowered. Can be useful vs evade stackers when it hits. Also a powerful launching move.
As for your other topic about Ultimates. I never use Demolition Bomber. It's PvE exclusive, it's damage is often matched/beaten by other skills critical hits, and it's not really useful aside from being extremely damaging when it crits and possibly a movement speed debuff during F7.