The goal of the challenge is kill all the Dark Elf Mages which only spawn each wave, before your Eternia Crystal is destroyed. The Dark Elf Mages may summon many skeletons to assist them. They originally only spawn from a few doors, but will eventually spawn from every door in the map.
The mechanic of this challenge is that all but two of the Dark Elf Mages are immune to damage, while the rest remain invulnerable.
Once either of the two vulnerable ones are killed, a new Dark Elf Mage randomly loses their invulnerability. Players must continue hunting for vulnerable Dark Elf Mages among all of them until all of them are killed.
On nightmare difficulty only one of the Dark Elf Mages will bevulenarable as opposed to Insane, Hard, Medium, and Easy.
There is an easy way to finish this challenge alone on INSANE with a Huntress, some Squire Spike Blockades and Strength Drain Auras from a Monk. A build order could look like this:
Before the Start of the First Wave, you should have enough time to place all the required Spike Blockades (red circles). As the comment says, you need to make sure that the enemies in the Southwest leave their spawn. To do that, you can't fill the room with blockades. The first blockade placed needs to be at the end of that first short corridor, where it's possible to walk past. The necromancers can't resist shooting at anything close enough to hit, so even though the skeletons will walk past it, the necros won't. After you have placed the Blockades, switch to your Huntress. After the First Wave, you should have enough time to place the Strength Drain Auras. The Auras will greatly increase your time to react, because necromancers will always attack your Blockades and the Auras will reduce the damage they take.
As the comment says, there is a pattern which you can follow and that can be repeated. There can be some circumstances when following this pattern (e.g. your Pet kills a Necromancer on the way) but you can still follow this pattern to the end of the wave. Just have a look at the Killcount. (I tested it myself and it worked well).
First Wave (3/6)
Second Wave (4/6)
Third Wave (5/6)
Fourth Wave (6/6)
The final wave has 6 spawn points, but the pattern is largely the same.
Another way to complete this is with a Huntress of at least level 20 (so you can use piercing shot) and place squire blockades not too close to the enemy spawn points. This way the wizards have to come out of their safe zones and you can snipe 'em down. For defenses, place Slice 'n Dice or bouncers, since the annoying skeletons do a lot of damage if you can't find the vulnerable mages (as explained above).