Try to get on top of a nearby Swamp Dungeon (or high (horizental) Swamp Tree - they are indestructible and try to fight the Abomination from there - they are not able to hit you on (almost all?) of the toparea of the Swamp Dungeon. Use either fire arrows or. as the better solution - hit them with an axe from the top (may seem scary but it should be 100% safe to do). If they run off wait a bit and then jump down to get his attention again and lure him back to the Swamp Dungeon (dont forget to get back on top... :) ).
As an Alternative, try to get the Abomination next/on a Surtling spawner firepit. They burn nicely - but it can be difficult to keep them there.
Another option is to ignore them but as you wrote that can be difficult to do (and depends on what you want to do in the Swamp).