You will have defend only the center crystal to leave the other 2 to be destroyed.
Make sure to have the auras, the buff beam and the reflect placed, then place as much wisp towers as you can (you can also place a shroom tower if you want to instead of 1 wisp tower)
1 Problem: On the upper crystal detonation your defenses might be consumed by the black hole, and you will have to replace them, but this way me and my friend somehow managed to clear nightmare campaign.

• 2 Summoners with 6 turret guardians (Damage (2x), Attack speed, Range) 
• 1 Warden (should stay in for more damage, but can be replaced after defenses)
• 1 EV (replace with whatever after placing defences)

We also used Adepts with Instant tower upgrades to level up the stuff around us quicker mid run, but our weapons only dealt around 100k
Our stats were around 2k and 100k life
No idea how this lowering of the defense/minion unit works but we had 55/51 defense units around the last few rounds.
REPAIR YOUR ORCS, and use Summoner Healing when needed!!!
Hide behind the reflect beam.