But you don't want to be charging up another Giratina every turn
There's also a reasonable number of "cast from your graveyard" mechanics at this point, including flashback, jump-start, and aftermath, so there are plenty of options available, although there aren't too many Commander staples that use a "cast from your graveyard" mechanic, so your power level might end up fairly low
Just make sure you fill your deck with instants and sorceries
Most decks have only one Mega Evolution Pokmon in the mix (Pokmon, not copies, which may be two), and using Serena guarantees that you draw the desired card quickly
Mndanos tu consulta
But if time is the limited resource that is spent flipping sieges, players will probably prefer a more meaningful threat to reward them for that effort and hopefully end the game sooner