XY
AB

X and Y are two level 1 Magikarp, ~12 Speed in rain

A is DeoxysSlow: 503 Speed, Agility/Trick/Sleep Talk/Trick Room @ Choice Scarf
B is DeoxysFast: 504 Speed, Agility/Rain Dance/Sleep Talk/Tailwind @ Choice Scarf

Turn 1
Fast uses Agility
Slow uses Trick on X

Turn 2
Fast uses Agility
Slow uses Trick on Fast

Turn 3
Fast uses Agility
Slow uses Trick on Y

Turn 4
Fast uses Agility
Slow uses Agility

Turn 5
Fast uses Agility
Slow uses Agility

Turn 6
Fast uses Agility
Slow uses Agility

Turn 7
Fast uses Rain Dance
Slow uses Trick on Y

Turn 8
Slow uses Trick on Fast
Fast uses Tailwind

Turn 9
Slow uses Trick on X
Fast uses Tailwind
both Magikarp move

Turn 10
Fast uses Tailwind
Slow uses Sleep Talk
both Magikarp move

Turn 11
Slow uses Sleep Talk
Fast uses Tailwind
both Magikarp move
Rain ends, Tailwind ends


...

Trick Room test: first 8 turns are the same as above

Turn 9
Fast uses Tailwind
both Magikarp move
Slow uses Trick Room

Turn 10
Slow uses Trick on X
both Magikarp move
Fast uses Tailwind

Turn 11
both Magikarp move
Slow uses Sleep Talk
Fast uses Tailwind
Rain ends, Tailwind ends

...

Trick Room test round 2

Turn 11 (Trick Room active)
both Magikarp move
Fast uses Tailwind
Slow uses Sleep Talk
Rain ends, Tailwind ends

----------------------------------------

Conclusion: >10000 Speed = 10000 Speed