basically a good build for magikarp.
i mean, wouldn't it be funny if you battled a magikarp in competetive and lost?
that was a rhetorical question.
i can't think of a good move-set for one though (joke).
seriously, however; if magikarp was your only choice, how would you train it?
Magikarp @ Focus Sash
252 Atk/252 Speed/4 Hp
Note that this is EXTREMELY unreliable but the best set it gets.
Hope this helps!