Hydra will spawn in this gap as you approach. Make steelleaf armor or whatever gives you fire protection for this fight. Shoot into it's mouths, -- and be aware you'll need to be fairly close for ranged weapons to have any effect. You'll do the best damage from deflecting it's fire balls back into it's mouth.
Minecraft version: 1.12.2. Twilight Forest Version: 3.11.1021. I recently researched the Twilight Forest mode a bit (I'm trying to build an adventure map there), and saw that most biomes there's some kind of weird acid rain. So in 1.16-ish versions of Twilight Forest, they introduced a new layer of customization with the Tags system in vanilla, instead of the liquid being fixed to Water, the Edges and such being fixed to Grass, and the "Decoration" being fixed to Flowers, it uses a Tag to determine if those blocks are valid, same case with the Catalyst Item.