There is a website that can show you where on your world seed the leviathans are if you don't mind finding it out in a cheaty way. Here is the link, you just need to input your world seed and then it can show you where they are by clicking on the box on the left of the screen that reveals where the leviathans are (https://valheim-map.world/)

I always farm the abominations by going to several different swamps that I frequent and looking for the green stumps that are just hiding there. If you shoot them when they are hidden as stumps with a fire arrow, you can do a lot more damage until they wake up and start coming for you. I think they respawn every 2-3 game days, that's why I travel to a few different swamps to farm them. Also, if you want to be fairly safe, you can hop up on top of a nearby crypt and pelt the aboms from that spot and they can't really do anything to you. Just be careful it's not nighttime because wraiths can get you when you are up there, not much of anything else though.