This class seems around the power level of Watcher, and likely stronger. I was easily able to kill the Transient, Act4 Elites, and Heart, on Ascencion 20, and still coming out of all those with max hp. Apart from the above, I defeated 4 other Elites and 1 Boss without taking any damage at all. That said, this win came after 11 previous losses when I was still learning how the class works, but in 5 of those I got to the Heart, and 2 others I got to Act 3 bosses.
The cards themselves are strong enough to carry you. The tons of buffs and debuffs that melodies give you for free are what really pushes the class to ridiculous levels. Being able to stack strength/dexterity and apply weakness/vulnerable repeatedly, and with relative ease, is very powerful. Also, the high cost cards are really strong, so Sneko Eye is extremely powerful on this character. The "Shifting" card is too good. A card that is able to give 32 damage, 32 block, or a mix of both, is incredibly strong. Magnum Opus is hilariously overpowered. It's a win condition by itself, but if combined with En Garde, it can practically one-shot bosses.