Im using two different kinds of grinders:
The first one is at my own base utilizing "Cursed Earth" from Extra Utilities and 3 MFR grinders in a row. This gets me all the brains, string, slimeballs (...) or gunpowder I could possibly ever need. Basically a 15x5 field of cursed earth and the three grinders do the rest. Energy in and items/mob essence out is done via tesseracts. But cursed earth only spawns the basic overworld mobs, so no witches, blazes and such. On the plus side, the grinders make short work of Endermen, too.
The second system is set up as a public spawner room on the server, this one handles the more.. specific mobs. As of now, it has T5-SoulShards for Zombies (
will be replaced with Angry Zombies one of these days), Witherskeletons and Blazes. The T5 for witches is in the works. Set up is using Soulshards to spawn the mobs, MFR conveyor belts to gather the mobs up in front of two CC melee-turtles that kill them. The occuring XP orbs that are caused by a turtlekill are gathered with a LXP gatherer from Immibis. This method kills mobs just fine but will not kill Endermen. For those, I have two stacks of quicksand in my AE system, which are going to be a drowning Enderender, once I finish the Witch-T5 Shard.
I could add in images if anyone is interested.