WAIT TO UPGRADE Until after the other 2 cores die.
I typically had 1 TB (8000 stat ab1) and 1 Boost Summoner (hunt+monk guards) out. With upgrader for first wave. Both had 90 res, because what are you doing without 90res.
3 owls, 4 mortars, 4 du buff beam, strength drain aura, ensnare aura, and a few reflects.
You can use more minions than I did, because I didn't place them until after 1st wave and the other 2 crystals had died.
Hint 1: Minions are blocking path to south crystal, not on the bridge above.
Hint 2: I placed the auras in that corner so it was just covering the crystal for first wave. Then moved them closer to the crystal for 2nd wave. (After the other two crystals had died.)
This build just worked for me the first time and was rather stable.