Activate only as a sorcery and only once each turn
Just to paint a picture, Thalia and The Gitrog Monster looks like a name that barely fits within its frame
If an Adventure spell leaves the stack in any way other than resolving (most likely by being countered or by failing to resolve because its targets have all become illegal), that card wont be exiled and the spells controller wont be able to cast it as a permanent later
With Mavinda in play, your instants and sorceries that target have a free flashback cost, and that can be used to blink, to cast powerful combat tricks, or to cantrip with a card like Defiant Strike
End Step The Masamune is certainly going into every deck with Sephiroth, just for the sheer pleasure of having him swing his iconic sword, not to mention its ability synergises perfectly with Sephiroth, One-Winged Angels emblem
Especially with discard