I don't think Max Power is any harder than Teioh or Star Breaker to beat, and I don't think you'd need a perfectly custom tailored choco to beat challenge 15. I have a fair weather, relatively high cunning choco that I frequently use to race the costa track with and the challenge wasn't difficult. Just pick skills that improve your choco's strength or cover its weakness, like super sprint for a low cunning + fair weather choco; recovery + heal for lots of stam; etc. If in doubt, you can always appeal to rng for a hero's tonic ^_^