As above said, good/cheap way is roll a Map with the Beyond Mod on it.
I usually use it on the "Arid Lake" Map as its pretty small and you can pull enemies together (which you should, see how it works)
Also a breach appearing on the map might come in handy, as it spawns a lot of monsters on a spot.
Heres how it works:
1. You Kill Monsters. There's a chance of a small red globe to spawn on the floor.
2. Four red globes close to each other (approx 1 screen) = Chance to Spawn Beyond Demons.
3. You Kill Beyond Demons. There's a chance a sort of Portal/Blood Fountain spawning on the floor.
4. Three of there Portals/Fountains close to each other (approx 1 scree) = Chance to Spawn one of six unique Beyond Bosses
You need to be at least on a Map/Zone with Monster Level 68+ to get Abaxoth!
Hope that helps, its a bit of luck to get especially him, and he's veeeeery tough. So be prepared :)