I Find that Faradrella seems to be useful here, her supportive skills are definitely worth it, and of course her ultimate (I can't remember if she can reach level 6).
There are always some way to provoke enemy to waste their powers.
Those Catapults must be dealt first whenever defending, Ancient's supportive ability to aid defenses must be utilized.
Make sure the Optional Quest is completed. This way you have Fulbolg shaman which is pretty useful as well.
Number one threat : Kathranis (his shield and that damn ice skill is WAAAAAAAY OP, making him top in the list)
I could say that Furion's healing ultimate is pretty good here (I'm certain we can reach level 6 here since I recall I reach it in Highborne's Storyline). Since I don't know how rapid AI in Insane is, I could assume that they attack like 2 minutes per wave in decent force.
Btw, since they're reliant to magic, one or two wisp Detonate can screw their day since Wisp takes a bunch of mana in a decent radius.
Target order :
Heroes (Kathranis > Illidan <damn Ice OP stun>)
Sieges (use Ranged for this, Melee is WAY risky unless they screw your base like annoying pests)
Spellcaster (just use Wisps for this if you want it easy, they're useless without spell anyway, still that Heal must be disposed for being a total annoyance)
Ranged (this can screw your Melee troops, iirc, Melee use Heavy which is weak to Frost Arrows)
Melee (have Melee distract them for awhile first)
Also, I can't remember but if Ancient of Wonder has Orb of Venom, buy them to slow down heroes.
I just wish NE has those Spell Breakers and such, surely that ruins Highborne's day