Assuming you have already trained your Pokemon to LV 71, it will be very difficult to EV train since it is ideal to EV train Pokemon whiteout any EVs, a blank sheet is required so to say. It would require a immense effort to calculate its current EVs, since there is no way to know what it currently have. I will therefore suggest 2 approaches.
Option 1. EV train a new Empoleon. The easiest option is definitely this, breed a new Piplup an EV train that one. Another major advantage with this is that you can get one with a good Nature.
Option 2. Reset the EVs: If you have BW2 or one of the Gen 4 games, you can reset your current EVs using different berries. This is advisable if your current Empoleon have a good Nature.
Depending on how much work you want to put into this, choose Option 1 or 2. Now, on with the moveset:
Empoleon @ Choice Specs
Trait: Torrent
EVs: 252 Satk/ 252 Spe/ 4 HP
Modest Nature
- Hydro Pump/ Surf
- Ice Beam
- Flash Cannon
- Grass Knot
This is an all out attacking set, being very capable of hitting very hard. Hydro Pump/ Surf is STAB and the rest is for coverage.
Empoleon @ Leftovers
Trait: Torrent
EVs: 252 HP/ 252 Sdef/ 4 Def
Calm Nature
- Scald
- Stealth Rock
- Toxic
- Ice Beam
This is a more defensive approach, being able to stall extremely well and set up Stealth Rocks.
EDIT: I made some seriously advanced calculations, and I have an approximate value of your EVs and IVs(these calculations were made assuming you have a neutral nature):
Stat: EVs: IVs:
HP. 0-116. 0-28
Attack 82-204. 0-31
Defense. 0-42. 0-31
Special Attack. 16-164. 0-31
Special Defense. 0-36. 0-9
Speed. 72-194. 0-31
