I disagree with you. Mainly because the Knight Enchanter specialization *isn't* overpowered. If it was, you would be able to solo the game with just it; you wouldn't need any other skills from other trees. Personally, though, I haven't heard of anyone soloing the game with only the Knight Enchanter tree, so either it simply hasn't been done yet (something I find hard to believe), or the specialization isn't as powerful as people like to think.
oh but you can even on nightmare...
just get barrier, with spirit blade you get pretty much invincible (30%of your dmg converts to barrier strength) and do even more dmg to shield or guard so if any enemy pops it, it turn that monster into healing potion. Get some masterwork with guard on hit and most important heal 25% od dmg over 10 seconds which makes you immortal. I only use spirit blade and occasionally missile barrage (or how it's called). You can also use frost mine and put it slightly behind you so you get another 50% dmg reduction from frost armor passive. And that's pretty much it, you can solo dragons with it, but it need some practice cos dragons can be hard to hit with melee attacks.
just get those spells - teleport (dash you forward), barrier, spirit blade, missile barrage and fireball (great against stealth opponents - if you hit them when they are in stealth they appear and you can see their siluete moving around and if you miss you won't lose mana) and then get as many passives as you can. But don't pick the one which makes half of your barrier disappear to empower fire spells.
for the OP and the staff dmg type. If doesn't really matter cos spirit blade dmg is physical and it only adds 1/4 of the dmg from dmg source of your staff but if you want best staff, pick electricity and keep spare either fire or cold in inventory so you can change when fighting tough specific opponents.
and for the companions pick either Blackwall or Cassandra with shield spec, Sera (her spec is overpowered as hell - cost free abilities with no cooldown for 8seconds...) with dual blade and whatever mage you like for extra barrier and you pretty much won't need potions at all (give everyone 25% heal over 10seconds, guard gain on hit and disable potions for everyone but tank) and even if some other character get hit, don't hel them after combat, they heal on their own)
this setup if working like a miracle for me on Nightmare just get lvl 2 and then lvl 3 weapon as soon as possible, the game is really easy and enemy UI is stupid