girantina is more effective with weed out card, or with a combination of parallel city, skyfield in expanded. evolving cofagrigus or hydreigon is the problem here though. it will be 3rd or 4th turn before attacking.
perhaps a deck with cofagrigus, weed out hydreigon and girantina in it is more useful, needs sky field for it to work, assuming you can get 3-4 girantinas on the bench using battle compressor, acrobikes, mysterious treasures, or ultra ball, more effective in EXPANDED FORMAT, you need to fill out your bench with exeggute or more cofagriguses. this deck is pretty cumbersome, but definitely not good enough to be a good deck. possibly parallel city this turn, after using girantinas effect and then revert back to sky field next turn. Right now theres very few cards that can discard your bench.
sigylyph can instantly knock any gx or ex pokemon that can kill it one hit, combined with bursting balloon, rock guard, or if it attacks firt choice bands. vulnerable to non-gx or ex pokemons. useful to put fury belt, or use shindja to lower prize count.
if your lucky, girantina can instantly knock out 1-2 pokemon with 30-40hp on your first turn, if you can use battle compressor first.