Boss: The Twins

Located in Essence of Betrayal

Best training area:

Essence of Suffering-2

Strategy: Start on the highest level and lure all the monsters to the left. When they are close enough, just make them fall over the edge by standing or jumping near it and all the monsters on the highest level will eventually be on the second highest level. After that, repeat the process to get ALL the monsters to the lowest level. You may need to jump to get the monsters to continue following you. When all the monsters are on the same level, you can use an attack like Scar to attack them all at once and just sit back and relax while they're all slowly dying; or you can add in some attacks to speed things up as well. When they're all dead, I'm sure you all know this already, but you can just exit the map to the right and then re-enter to get the monsters to respawn immediately and resume training.