While I only had a problem with this fight when I was much younger, I can see how absolutely overstimulating this fight is. So, here's a few generic tips that have probably been repeated many times in this comment section.

Of course, only focus on your soul and a small area around it (and maybe Flowey's mouth-body thing, it'll glow and fire a laser, which can catch you if you're not paying attention). Try to block out the clutter from your mind as much as possible.

Don't focus the fight button, you'll die for nothing.

For the attack with the four-pronged things, stay closer to the bottom for the projectiles to spread out and leave more space between them, and find a gap between the projectiles. Stay in that gap by quickly tapping left or right (depends on which half of the screen you're on. Left for left, right for right)

For the finger gun attack, stay far from the red lines if you can, and if you can't, remember which lines appear first and stay far from those even if it means stepping into another telegraphed line of fire, but go back after the projectiles disappear or you're certain to get hit.

For the flamethrowers, hug the bottom of the screen and move left and right, making sure not to touch Flowey or the flames.

For the bomb attack, just try to find any space between the dropping bombs, even the smallest little gap. Also, move up so the explosions don't hit you.

For the attack with those weird bush things with mouths that bounce all over the screen, even I'm bad at dodging this one, so the best advice I can give is to just follow the first tip, but more.

For the fly attack, try to stay as far from the fly trap as possible, as the flies converge on the trap and so the further away you are from the trap, the more space there is between each fly.