DECK PLAN:

Deck plays evil minions, and mortals to Shapeshift/Sinister pond into them.

If you see Mephistopheles, after ramping to 6 mana you can ditch avatar and masks and you finally discover your true nature and stop pretending to be someone else. Be sure to exhaust some mask before doing so - Archimago and Sorcerer should arguably be played before Mephistopheles if tempo allows it. Also Avatar of Air for early game removal without expanding a card. And most importantly Pathfinder for ramp if the tempo allows it, if not, can skip. Other masks are situational and very expendable. In truth you want to cheat him on board, this is last resort or very late game play, not having access to ironclad mask in case of swing to door can feel bad.

EARLY GAME:

Early game you need 2 air threshold. If you see starting hand Tower, another air and one water, you are golden for the get go. Two Air and one colorless is also fine.

Early mask into pathfinder with Tower is optimal, as not much damage ping are available to opponent for 1-2 mana. You really want to ramp to 6 as fast as possible in this deck. Optimal is also highland falconer with Tower. And early game pressure, so you can go to Pathfinder after baiting removal with pressure. Now if someone knows how to play against imposter and comes close with Avatar, you punish them with Interrogator and for some card draw or early game swings, to make them go more on the defensive.

Mist of Doom

@treexnerd