I'd put this in Cross-Platform, but the most basic way is just to build a giant corridor 70 or so blocks left and right in the Underground Hallow, with a volcano trap (a pit with smoothed sides and lava in the centre) in the middle and you sitting under. Just put one lava bucket in a 7-block wide hole, and use an actuated passage to get in and out. Just sit under the volcano trap and set up a Water Candle for maximum spawn rate. If you want, setting up a dart trap either side of your volcano will activate any Mimics which will get you even more cash and items. However, Chaos Elementals (the only enemies that drop Rod of Discord) are a relatively rare spawn, and it's only a 1/500 chance you'll get a Rod anyway. It can take upwards of an hour to farm for one, so stay patient and don't leave your box and you'll be okay.