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?
yours indubitably.
Magikarp @ Focus Sash 252 Atk/252 Speed/4 Hp Hasty/Naive/Lonely/Naughty/Rash/Mild Nature -Splash -Flail -Bounce -Hydro Pump
Note that this is EXTREMELY unreliable but the best set it gets.
Hope this helps!