May i suggest Clamperl? With the addition of Drizzle and Shell Smash to work with, along with the Deep Sea Tooth, it can hit 984 Sp. Atk, and thats only with a timid nature, along with a 30 IV for a Hidden Power (Preferably grass)
Clamperl @Deep Sea Tooth
252 Spe // 252 Sp. Atk // 4 HP
Timid w/ Rattled (Raises Speed when hit by Ghost, Bug, or Dark-type moves)
30 Atk. IV // 30 Sp. Atk IV (For HP Grass)
~ Shell Smash
~ Surf
~ Ice Beam
~ Hidden Power
This pokemon is remarkable, although it's speed can say otherwise. I've swept with this thing plenty of times, after eliminating Choice Scarf users. Clamperl hits a respectable 358 Speed, factoring in the Timid nature, and hits 477 Speed if Rattled is activated, which outspeed most pokemon, including 100 base speed scarved pokemon with netural nature, and can at least dent anything not named Ferrothorn. Speaking of Ferro, here's a calc of an Ice Beam Vs. Ferrothorn, different variants as well.
+2 DST Timid Clamperl vs. 252 HP // 168 SpD Ferro = 61.6% - 72.5%
+2 DST Timid Clamperl vs. 248 HP // 8 SpD Ferro = 70.9% - 83.4%
Even with Ferrothorn being one of the only pokes that can stop it, if it decides to Power Whip and misses, its gg. Either way, 984 Sp. Atk Rain Boosted STAB Surf will wreck anything unprepared.