If you've got Rotarycraft, it's spawner controller takes power and generates ludicrous numbers of mobs. True, it's a bit tricky to power, but once it's going, there's no stopping it.
IMO redstone-lamp-stoppable cursed earth + grinder is the best.
To get the sigil I just toured a couple desert temples and found a runic dungeon chalk, and went to have some fun killing mobs and finding loot (the sigil).
Also awesome to get tons of spawners, bring many diamond dollys.