I struggled insanely...Fusion Flare didn't work like it had on Zekrom, getting me off to a bad start. In the end, Samurott took it out with a critical hit from X-Scissor, before taking out two pokémon with Torrent-boosted Surfs, and then being sacrificed for Reshiram to do the rest.
However, I don't know if anyone else agrees with me, but was that not just the creepiest battle ever? The dark background, scary-looking pokémon such as Hydreigon, and the creepy music (or lack of it) all made it absolutely terrifying. Honestly. I guess the way Hydreigon just obliterated 5 of my pokémon without taking damage made it even worse...but whereas N's battle was epic in the way you have legendaries smashing the crap out of eachother, then a load of strong pokémon battling their hearts out for two of the most compassionate trainers in the land (including, in my case, a Zoroark showdown), Ghetsis' battle was like a tense pokémon thriller.
Then again, that's just me. And above this sentence is a wall of text.
Oh, and by the way, unlike Gothitelle, I did it with the battle style as "set", not "shift".