This is the kill room for the Gaia Guardian:
11x11 with the summoning platform in the center and Diamond Spikes towards the sides and the back of the room. Fans in the walls at the front and both sides. Summon the Guardian (needs an ingot crafted from Terra Steel and four Gaia spirits). He ports around a bit, but normally stays in melee range.
Get him down to 25-30%, that's when he starts to summon all the zombies, skellies and wither skeletons. You get advanced notice, because his health bar turns grey. Run out of the room and step on the pressure plate right behind the entrance. This activates the fans, all mobs get pushed towards the center back of the room and killed by the spikes. It takes around 20-30 secs, then his health bar turns red again. Run in and finish him off.
The Guardian drops dice, which you can turn into relics. Also Mana Steel, pearls and diamonds (means pretty much free Terra Steel ingots), enchants for the helmets and some other stuff. Especially useful is an item, that turns the completely annoying infernal Wither Boss into some pink wuss with normal health and no special abilities. Makes getting Nether Stars a lot easier.
The last couple kills I didn't even see the health go down anymore. Might have something to do with wearing gear like this.
And I still need to kill him some more. Still missing a relic that supposedly can be used as food and he drops Overgrowth seeds, which double the efficiency of Botania flowers.