If an effect instructs you to transform a modal double-faced card on the battlefield, it transforms only if its other face has a permanent type (that is, if its other face isn't an instant or sorcery)
All you need to win is a creature with power 20 or greater when the final part of the ability resolves
So if they give five open mana, you can just exile six cards to make sure your opponent wont be able to resolve their spell
It brings to life in the physical world the magical lorebook from the game (represented by the card deck), which is a cross between a home of sorts for the glimmers and a magical journal and field guide for Illumineers
Any deck that can play him is going to have something of an advantage, as he makes it possible for you to play powerful songs for free
Typically this is the kind of thing I keep in the board for reanimate decks or some other archetype that can produce something before affinity kills it that four damage can't handle