No ubers, no NFE's, level 100, max IV's, neutral natures, no items, no EV's
Opponent switched in B!
A used X.
???
B lost 4% of its health.
A used X.
???
Critical Hit!
B lost 49% of its health.
B used Surf.
A lost 26% of its health.
I'm fairly sure there's only one solution to this...hopefully...