So, still not much in the ways of success. Was able to optimize the Giratina fight, though! So now I don't use any items there. Also managed to get a little better in the Arceus fight item-wise, though the amount of FP Drains wasn't great, I entered the last phase with more items than usual.
Then, the last phase kicked my ass again. This time, I was prepared on what was going to happen though - just due to poor blocking and move order I had to use Galaxy Star earlier than I'd have liked, which left me struggling to keep up after the nuke, and in the end just lost due to skill issues.
Thankfully, this means I don't have to change my strategy much! Just get better at blocking the really important things. Next stream, another attempt at it.

For those who may care, I'm trying to reach affiliate! Admittedly I don't know what time I'm streaming anymore, but the goal is to do so almost every day. I still need 18 followers for affiliate, so any little bit helps! www.twitch.tv/bolttheblueemolga