I'm going to post this here in the hopes that it gets popular, powercrystals hears about it, and nerfs it.
Find a vanilla slime. Use several slime embiggening syringes on it. Bit tricky, you have to be close enough for it to damage you. You can only get it up to the size one larger than the largest one that spawns naturally, size 8(the size doubles with each stage, 1, 2, 4, 8..., try using a bukkit plugin or something to spawn a 128 size slime for fun). Capture that with a safari net. Build a nice little enclosure with an auto-spawner in the middle and grinders on each side(best have the walls 4 or 5 high, the big slimes can jump more than the small ones). Put the size 8 slime in the auto-spawner, set it to exact mode, and supply power and some initial mod essence. It spawns a big slime, the grinders kill it, it breaks into 2-4 size 4 slimes. But the grinders get mob essence from that. Pump it to a tank, then back to the spawner. The grinders will kill the size 4 slimes down to size 2's, then size one's. And the auto-spawner spawns more big ones.
If I remember my calculations correctly, you get nearly 8 times as much essence back than you put in. It just takes a fair bit of power. And as a nice side effect, you get way more slime balls than you could ever know what to do with. You can smelt some of them into resin so you never run out of rubber, but I have 30k+ in my AE system and I'm going to start voiding them.
Hope that helps