Holy crap. This challenge SUUUUUCKS. There is so much that can go wrong. On my completion run, the left core was one hit away from dying.
This is the build I used to beat the challenge, but it's very inconsistent because of weird mob shenanigans.

This build uses two characters, the first is a DPS, I'm bad at wheeling so I used a DPS app with a Moon Staff for Mana Bombs, but if you're good at wheeling uise a jester with a Moon Staff. The second is a Tower Boost Monk to keep the auras alive. Tower Boost Monk goes at hint 1, shooting towards hint 2. On the earlier waves, whenever the buff goes on cooldown, switch from the DPS to the Monk and upgrade, then switch back when the cooldown finishes and boost is back up.

Build 1: Do both aura stacks and the buff beam, the beam should be able to reach both auras.
Combat 1: Collect mana with Tower Boost Monk to upgrade the buff beam. The beam MUST be max by the next combat phase, and you have very little time during build phase, so it's better to do it now.
Build 2: Build every golem and every reflect you can. You won't be able to do all of them, but you must have all golems except hint 3, and at least the reflect on hint 4 done by the end of this build phase.
Build 3: Finish the build, it must be finished by this build phase.

The reflects are required for the final phase, otherwise the golems will die to fireballs. The skeletons don't deal much damage, but the mages hit the defenses like a truck.

Some enemies will somehow manage to get past the golems. The skeletons will be knocked around by rocks/chickens, or get stuck in death and resurrection animations, and won't be able to attack the crystal, and the mages should aggro onto your Boost Monk. If a mage aggros onto your Monk, they don't deal much damage, but they'll still chip you down, so make sure to pay attention and heal whenever he's in danger.

Mages that spawn inside the bottom spawn on the bottom floor will routinely just sit inside the spawn and take no damage from player attacks, even if they're designated as a vulnerable mage. As far as I can tell, there's no way to mitigate this, it just slows down the wave if one or both of the mages are sitting in the spawn.

Golems are treated as minions for some reason, and can damage vulnerable mages. It helps a little, but you shouldn't depend on it.

Invulnerable mages will only display damage numbers (the number zero) from player attacks, while vulnerable mages, despite being immune to defenses, will also display damage numbers from defenses, even if they're also zero. If a mage has zeros flying out of them at the rate that your lightning aura deals damage, it's vulnerable. Again, only helps a little, but it can help to figure out if a mage that has gotten past a golem is vulnerable or not.

Other than that, pray that not too many enemies get past the golems, and not too many mages sit inside the spawn. You might be able to use minions to block up the sides, but I didn't have enough build time to place minions at all.