I built a three row farm to grind out the Rod of Discord. Center row is a mob grinder pit which is just a volcano shaped hole with lava in the bottom of it to burn everything that falls in it. Below it is where I stand while it works. I also have dart traps on each end to trigger any mimics that might spawn on the row. The other two rows I have actuated fire traps to burn through the mobs and it usually takes care of the enchanted swords as well. Above and below that farm is dug out 40 blocks and filled with lava to prevent outside spawns and the rows run 60 blocks left and 60 blocks right. Chaos elementals I think is a 1 in 500 chance to drop the Rod of Discord but luck plays a huge part in it as well. With that said I have farmed for it many, many hours and still haven't got it. I have managed to get over 5000 souls of light, 7 Chaos elemental banners, 2 enchanted sword banners, a nymph banner, and a crap ton of mimic loot, key molds, and mob drops.