I am not sold that adding +5 or higher is the answer. Given how the game works, anything higher than +6 chances are you will never be able to hit it, let alone cause any damage. Seems to me this is leading more toward a stalemate than anything else.
I cannot speak for anyone else, but most all of my fully decked out characters will solo bosses and EBs rather easily whether there is one or ten of them. Adding more HP and ludicrous spike damage to NPCs is the easy answer, but to me this is not adding challenge, just drawing out the battle.
Were I a real choice, I would choose a more intelligent AI that can adapt to you.