Workshop is currently disabled on Steam, because Steam demanded that the developers moderate the workshop for copyright infringement, and developers don't have the manpower for that. They'll probably hire somebody to do it, and then reopen the workshop. But this isn't going to happen immediately.
Also, workshop was very short-lived, so you shouldn't expect anything from it yet.
Bring everything. Healing potions, shields, allies...
Allies probably wouldn't last long in an AoE fire, but they'll buy you time, and probably do some damage too.
You probably don't have the best shields yet, but maybe you can sneak in and get a winged workbench for winged shield to be faster.
Healing potions, on the other hand, are a lifesaver. Get at least 10 of them.
As for the fight itself, the important part is to stay mobile. Don't commit to heavy attacks, or you might spend too much time in AoE fire. Use light attacks instead. Whenever possible, attack the gunner - he is the one with all the weird tactics. And run away, because he is sure to put fire where you are. When you make mistakes, use a potion.
Katana is a good weapon for that fight, but I think you can't get it without cheats before that fight, as it's in the dungeons that are locked beyond the treasure hunters fight. So you'll need an iron sword or a battleaxe.