To manipulate Bowser on framerule 538, stomp on the first of the two goombas during the hallway.
To make the double-bounce on the koopa more reliable:
You need to play this level fast to get into the good pattern range.
Do buffer jump strat at the start, this is required to manipulate the podoboo cycle. Otherwise, you will not get past the podoboo.
Swim very low before the hallway to manipulate the firebar, this is easier to do on 8-9/A rather than 8-B.
If you don't manipulate the firebar, you probably went too fast or had bad movement in water room. At that point just close-out a low-7:56
Ideally aim for Frame 36 or Frame 38
Perform a fast accel (doesn't need to be perfect, just good enough) and slow down by a pixel, by doing a semi-fast wiggle at Dual-Koopa section