Abomination

The Abomination is the swamp's walking tree, and the main reason you will farm the biome's muddy trenches: it is the reliable source of root, the material for the root armor set. Fight it with fire. Its blunt and pierce resistances make melee a slog, and it ignores chop and pickaxe damage entirely, so bring a torch, fire arrows, or a weapon with fire damage rather than your axe. It drops guck as well, which stacks up nicely, and the trophy is a solid build piece if you get the drop.