Big Bang, an useless skill?

It's very situational. Like you said it's only useful whenever you can precast it like jumping down or while you're waiting for pap to wake up.

So, i think add 1 point in it is more than enough for this situation.

Ice/Lightings just lol with it.

Kinda agree with you. Being a 4th I/L, i found out i use CL more frequently than all other skills, although i had some point in my BB here.

Sometimes i will face a bunch of mobs at the bottom, but i rather choose ice strike instead of BB to kill em.