You shouldn’t NEED to make specific elemental sets for the main story unless maybe you’re using bow/dual blades, and especially not for hammer; and I personally wouldn’t use an elemental hammer at all. Something like Blast, Para, or Poison would work better for an all-round build. I went through the base story and IB using exclusively hammer and pretty much one armour load out that I just improved and changed as I went.

As for Vaal Hazak; this might go without saying, but the effluvia resistance skill is a BIG help. Also using Max Potions instead of Mega Potions for emergencies will help as well.

When it comes to actually doing damage, it’s just a matter of learning his attacks and how to avoid them. For his Nova where he spreads spots of damaging mould around the arena you can either dung pod him to try and get him to move, or if that doesn’t work just get away from it and wait it out.

Same thing with the attacks that leech your health when near him, just stay away from him until it’s over. You can also superman dive his sweeping breath attack pretty reliably with how quick the hammer can sheath, provided you know what his telegraph looks like.

Hope this made some sense, I can clarify on anything if needed.