If your swamp area has fire vents, then you can use them to kill abominations. Kite them over the fire vents, setting them on fire, and let the ticks of fire damage do most of the work.

When they're not on fire from the vents, shoot them with fire arrows to keep the fire dots going.

You can use a hoe to lower the ground around fire vents so the surrounding water will just barely cover the ground. The surtlings will die instantly as soon as they appear, leaving piles of coal and cores everywhere.

I have a whole carefully manicured swamp area where there are 8 fire vents in a really small swamp area. I just roll through until I see an abomination stump (they're smaller then the regular tree stumps, and brighter green). With a good bow, you can knock a HUGE chunk of it's health off just while it's doing it's animation to crawl up out of the ground & flip over. That animation is insanely long.