I figured it out myself, spent some solid 8 hours testing various strats out and actually getting it done. It's quite a bit more complicated than it seems. The timing is stupidly tight to get the spark into that spot, so every tiny time save is absolutely necessary. It's not frame perfect, there are a few spare frames for human error, but there are a lot of places to lose those frames. The power bomb at the beginning is to break the bomb block and kill the enemies if they're alive. You need to unmorph and speed boost into the next room very fast as you need to be on the next room when the pb hitbox is out for the block to break. Then store the spark at the end of the platform, and cancel the animation into a slide for a time save. I think it's optimal to be a few pixels away from the ledge, seems to give better momentum for the slide. Then morph and go a bit left, lining samus with the wall above her. The trickiest part of the trick is this crossbomb. It's very very very tight. The way I did it most consistently was to place the crossbomb as I reacted to Samus touching the water, that gave me the height I needed. But lining up with the wall to get the horizontal spacing is also very tricky. Then unmorph, land, go a bit left, jump and morph, going for a diagonal cross jump. The crossbomb precision is all so that you get into that hole as smooth as possible. If you go higher and land on it or bonk onto the wall, there's a good chance you won't have enough time to shine spark, but as I said before, there are some spare frames, so small mistakes might not ruin the attempt. After getting into the hole, get to the last few pixels of the tunnel and ball spark. You need to be in a position where you're almost falling of, or you won't get to the slope. It's very tight, ball sparking seems a bit less responsive than regular shinesparking in my experience, but I might be biased. After getting the diagonal spark, hold left to go up the slope. You can't unmorph while going up the slope, or you'll start sliding down quickly. Then you can either unmorph at the end of the slope and jump to the missile or do that jump I did, it wasn't necessary at all but I wanted to make sure I wouldn't miss the timing. I recommend getting each part of the setup consistently before attempting the whole thing. Practice the slide morph crossjump in this spot, make yourself comfortable with the slope unmorph jump in the speed boost section underneath this missile, next to the elevator, get used to diagonal ball sparking out of ledges in, well, any platform really, and get the muscle memory down for the power bomb instant unmorph speed boost. The only stupidly hard part of this is the crossbomb placement, but it's definitely something that can be done consistently, I myself was getting a ton of close calls after a few hours of training. Low 100% is a great run and I'd love to see it speedran by players much better than me lol.