Play as Inferno.

You see, I already said that we've been playing in late game for 4 months, on a map where there's a mentor and you can level up to 30. I don't know where you read something else. No, if you mean that you're not interested in Creature Abilities (CA) and you level up differently, then it's a different story. But here, demons can't do without CA, because other races will also use the mentor. And what's so bad about CA? Even those perks or skills, like the ones there, are initially useful for demons; there's so much good stuff – surprise attack, battle frenzy, although I would prefer if there was also Mind Control and Tactics, but you can't do anything about it. You're just saying that demons have no chance, and I'm saying that they have a little bit of a chance.
By the way, I know what Purification is. It's when a spell is removed upon being hit by Adepts. Well, you can block them or take them down right away so they don't interfere, because there won't be many of them.