If you want more of a challenge, do some (or all) of the following:
1. Play on Nightmare difficulty
2. Enable Friendly Fire
3. Either don't use or severely limit healing / regeneration potion usage
4. Don't use items that give any elemental resistances (Avoid the Temple in Forbidden Oasis where you "redeem" the Shards)
5. Don't use / craft items or use abilities that enhance (or restore) health, barrier or guard or in any way reduce damage you receive
Knight Enchanter is not the only class that is arguably OP. And, Knight Enchanter has already received two Nerfs. Spirit Blade no longer Combos and Disruption Field doesn't work on some enemies anymore. Even a 1H + Shield Warrior, with decent equipment and properly spec'd (generate guard, armor bonus (bugged?), take less dmg from front, can't be flanked, etc.) will do very well even against a Dragon. You could also make the argument that Reaver and Tempest (Flask of Fire / Lightning + Thousand Cuts is also OP). This YouTube Video demonstrates Flask of Fire + Thousand Cuts vs Dragon on Nightmare. The Dragon loses more than 50% of its health. Even a properly Spec'd Rogue with a Bow can do very well against a Dragon. In this YouTube Video, Sera uses Flask of Lightning + (what appears to be) the Longbow of the Griffon, doing a substantial amount of damage to the Dragon. IIRC, the Longbow of the Griffon fires 3 projectiles.
I only managed to kill my first Dragon (the Storm Coast) last night. I wasn't planning on fighting the Dragon, but had the quest to deal with the Red Templars on the Storm Coast. After finishing that, I decided to go to Dragon Island. I play KE on Normal difficulty. I was trying to "cheese" the Dragon by attacking at range, but that wasn't working. The Dragon started to spit lightning from range and managed to damage and eventually KO my other party members. I used 1 health potion myself. My other party members had used the rest, so I had none left after that one. I eventually managed to kill the Dragon after about 20 minutes. I was level 16 and the Dragon was level 19. I had to keep on my toes and even used Mark of the Rift two times. The Dragon was still alive. After the Dragon was finally dead, I felt like I had accomplished something. I hardly felt "OP", at least not anywhere near what you see in the YouTube Videos. Granted, if you spec KE a certain way and take other passive abilities you can be very difficult to kill. But the same goes for other classes. You are not, however, invincible. Personally, I fail to see the point of Nerfing anything in SP mode. If a player believes something (a Specialization, Ability, or even Item) is OP, perhaps that player should not use what he considers "OP".