My Big Bang Hero build

Going by my playstyle today, I generally use Brandish even if I'm facing a mob of more than 6, unless they are very weak, in which case I use Shout.

After BB, the damage done by Brave Slash is 630% on each of 3 targets, which is more than twice the damage per target done by IF'ed Slash Blast (250% on each of 6 targets). So unless the enemies are weak enough to be 1-hit with SB (in which case they are likely weak enough to be 1-hit by Shout as well, 220%), it's faster to kill them 3 at a time using Brave Slash.

You mostly think the way I do so yes in most cases where most mobs would easily die with 1-2hits, you'd use brandish (brave slash later on of course) to kill targets. The range of brave slash is also slightly longer than brandish which helps to get there to mobs easier.

Soul Driver is slow. Slash Blast provides 6-mob pinning, which is the only reason to possibly use it. It's completely viable to use only Monster Magnet for this role, but Slash Blast does 50% more damage is all. It's hardly game changing in my opinion. Soul Driver will draw a lot of damage towards you, so it's more difficult, but it can be used in conjunction with Monster Magnet's long stun, provided it's uninterruptable to deal better damage. Monster Magnet makes Shout practically useless, unless you want to stun things behind you, which doesn't sound very useful at all.

Drawing dmg to you should be the least of your concerns really when big bang comes. You won't take anywhere near the dmg you currently take from mobs so that isn't even a problem anymore. As for the range of slash blast, the distance that you can hit is way too short for you to even decently hit anything without having the need to jump into mobs all the time and hope they bunch up neatly for you to actually keep pounding those 6 hits at them. I'd go back and use brave slash before considering the use of slash blast any day.The only time that you can hope you'd decently get those mobs is when you rush them all into the wall and even then you'd have to hope that none of those mobs get past you (whether that be due to your slow atks or them somehow just moving behind you when you chance a kbed state). The range of slash blast is the same as you using your regular weapon atk swings practically with stabs, swings, and slashes.

Take these examples: (The example on regular swings are based on 1h builds though, keep that in mind)
This is a stab: (regular swings are already unstable in range so I just took one)
Brave slash:
Soul Driver
offtopic but heres charged blow too

Would you really want to use that kind of range constantly? You'd barely hit anything, most of the time only hitting a few targets at most. Another use for soul driver is being what some people bash dawn warriors about: it being a "mini genesis." It does high higher height range than any other skill you'd normally use so it does have its advantages. If you ever tried a dawn warrior at gallos, you can actually hit both top and bottom mobs with just 1 Soul Driver which is quite useful there. Other good thing about soul driver is it hits behind you as well so you don't need to turn much.Soul driver is slow alright, but its about the same speed as shout and maple warrior. 250% x 4 is good enough for it to make up for slow animations among having a decent aoe range.

Concerning maxing soul driver itself, I need to go look around for it again I had something earlier with SSes but I lost it and have to go diggging it up again, I'll edit or repost if I do if I can find it somewhere =S. I was sure I saw someone putting evidence to maxing it without having the need to get 10 lvls every time to get 1 sp into soul driver.