I'm pretty new and not too good with core building yet, but this is the core I run in my Pokemon Showdown team.
Greninja @ Choice Scarf
Ability: Protean
EVs: 252 SpA / 252 Spe / 4 SpD
Nature: Timid (+Spe, -Att)
-Scald
-Dark Pulse
-U-turn
-Ice Beam
Ferrothorn @ Rocky Helmet
Ability: Iron Barbs...