At the start I gathered and analyzed data on the Gear-Yo by choosing it from DeMille's Gimmick menu then pressing Start to enter a practice mode. I'm sharing my autism.
To red zone press A when the light blue bar right of the dark blue bar overlaps the red bar. Both blue bars extend and accelerate together. The light bar preindicates by 1 frame the position of the dark bar.
The Gear-Yo has 2 attack stats, X and O. I assume X misses while O lands as those appear on fail and success. X starts at 21 for each difficulty level (L).
L[1-4] change attack (O-X) some, your frame window a lot (L[1]'s 13 frames are 2.6 times as lenient as L[4]'s 5 frames), and attack (O) a little.
L[5] is unique, doubling leniency from 1 frame to 2 on the blue bar's return so on L[5] you should press A on return, except L[5]'s only gain on L[6] is that undershooting the 1st O-frame of the return by 3-4 frames, or the 2nd by 4-5, red zones. It's accidental and, if off that much, L[4] or lower is better. Otherwise, choose L[6-7]. L[5] is least justified.
L[7] is frame perfect but its O-X surplus damage doubles L[6]'s O-X, with L[6] on a 2 frame window.
Your red zone % at L should exceed (O[L-1]-X[L-1])/(O[L]-X[L])*100 (the purple dot left of yours, divided by yours, times 100). Repeat for each L except 5 until true. For example, at L[7], (O[L-1]-X[L-1])/(O[L]-X[L])*100 is (36-21)/(52-21)*100 or 48.4%. Predict your red zone %. If under 48.4, drop level then recalculate. Repeat for each L except 5 until over.
The red zone % for mean target damage D is -(100*(X[L]-D)/(O[L]-X[L])). O[L]'s the blue dot, O[L]-X[L]'s the purple dot. For example, at L[6], -(100*(X[L]-D)/(O[L]-X[L])) is -(100*(21-30)/(36-21)) or 60%. Red zoning 60% at L[6] averages 30 damage.
I chose L[6] between L[4] and L[6]. I didn't double-check calculations or battle, and damage is probably reduced in battle by formulas with parameters like monster defense. But it's a game about tomatoes, not a math symposium. I'm just contributing to the Game of the Month thread.