Read the achievement description for "Archmage" -- I locked a thread on this exact issue yesterday and explained the same thing. Not sure why everyone is so keen to call him a cheater. I think if he was going to cheat after all this time, he'd do a better job of it.
That's not possible... You can't get to level 20 without doing any quests rofl. It's impossible to grind there, because mobs in a specific area are the level you first enter that area as, ie. if you enter, say the Wilds at level 5, all mobs there will be level 5 no matter what level you are.
Please read the submission rules again. We can only add new games if we have the xbox.com link or a link to someone's profile that has played the game. All the text lists you're submitting are useless.