If you're using a special set, I wouldn't go for Aqua Jet. too weak backed up by its 80 base Attack. Refer to Smogon's BW writeups (like this):
https://www.smogon.com/bw/pokemon/empoleon/uu
or just google "(Pokemon name) Smogon forums" so that you get a result like this:
http://www.smogon.com/forums/threads/empoleon.3496385/
Greninja also far outclasses Emp in an offensive role so you may want to use Emp as a wall of some sort. Here's what I pulled from the second link:
Empoleon @ Leftovers
Ability: Torrent
EVs: 248HP / 8SpA / 252 SpD
Calm Nature
- Scald
- Stealth Rock/Defog
- Toxic
- Roar/Protect
+ If you have trouble with SPunch, Will-o-Wisp will wreck them every time unless they have a Sub up.