Align the camera straight with the tunnel and longjump up-left in the tunnel, hold upright for the second longjump and keep holding upright on the brown slope.
Do a quick single jump on the brown slope to get higher up, then A+B jumpdive. This makes it so the flyguy will not troll you.
The landing spot is important, you do not want go to far back or forwards. (See Video)
After rolling out go neutral and do a full height single jump and then kick.
Double jump and nudge slightly towards the water before you groundpound.