Hello, for this challenge you need a lot of nerves. I found this strategy to work for me better than using golems.
I did this with jester using a aoe staff (The downpour) and a wizard.
Just keep running between the spots where mages pile up and keep your map open so you can see if any mage takes damage so you can kill it.
Monk with honey spear got me to last wave aswell but was a bit frustated so i switched the hero.
Hint  1 is where a mage can fall down and run to the crystal and start attacking so its up to luck that they dont fall
Could possibly sell one stun beam for a nature pylon so auras are even bigger so mages stack up earlier.
GOOD LUCK and just keep trying, one run will succeed