Mekk-Knight Blue Sky is the most versatile monster in the Mekk-Knight archetype because of its ability to search numerous Mekk-Knight cards from the deck to the hand
Flavor Text Fighting Fire with Fire Ineffective, Reports Flammable Wizard Number 67 Language English Legal Formats Commander, Vintage Not Legal Formats Alchemy, Competitive Brawl, Modern, Pauper, Pioneer, Standard, Timeless Banned Formats Brawl, Historic, Legacy Rulings (11/10/2020) If the target spell is an illegal target by the time Mana Drain tries to resolve, Mana Drain doesn't resolve
Looking at this, there're two decks that come to mind that can potentially break it - Lightsworn Rulers and Hieratic Rulers
While losing your own permanents is a big hit, you at least get to keep your Demons
Creature - Elemental Giant Draw (CTRL to add secondary) Finisher Karlach, Fury of Avernus Whenever you attack, if it's the first combat phase of the turn, untap all attacking creatures
Regal Caracal Regal Caracal may not come cheap, but its ability to give all other cats +1/+1 and lifelink more than makes up for the cost