Actually, magikarp can be useful,if used right.
Turn 1: send out smeargle, use spore
Turn 2: Use belly drum
Turn 3: Use rain dance
Turn 4: Baton pass to Karp, opponent should wake up and send Karp to 1 HP (with focus sash)
Turn 5+: use flail
Edit: I should point out: don't use magikarp anyway, unless you want to troll people or have a dare to beat someon with a magikarp or something; Blaziken can do this strategy better thanks to Speed Boost