To me, Empoleon seems to occupy an important niche in that he resists every STAB move coming off Kyurem and has enough defenses to withstand the infamous V-create. Although he is still weak to both Fusion Bolt and Focus Blast, two common coverage moves that these Pokemon carry, Empoleon's natural defenses and the help of resistance berries means Empoleon can stay in on these two with surprising audacity.
Set name: Utility Counter
Move 1: Surf
Move 2: Flash Cannon
Move 3: Rest
Move 4: Sleep Talk
Item: Chople Berry / Wacan Berry
EVs: 88 HP / 252 Def / 168 Spe
I was messing around with this set a little bit earlier. Even without Wacan berry, this set will not always be 2HKO'd by Adamant Victini's Fusion Bolt, unless Victini is choiced. It is, however, 2HKO'd all the time by Kyurem's Focus Blast (assuming no SpA drop from Draco Meteor). This makes Chople Berry seem like the better choice all around. The EV spread aims to outspeed Adamant Victini after the Speed drop from V-create; if Empoleon switches into a V-create on any common Adamant Victini, it is guaranteed to outspeed and OHKO in return with Surf. Flash Cannon will 2HKO Kyurem. 248 Speed EVs will outrun Jolly Victini as well, but you lose considerable bulk and won't fare so well against Kyurem unless switching into Draco Meteor. Outside of these two Pokemon, Empoleon notably also checks Fire types (including Arcanine) quite well. Particularly in the case of Arcanine, as Wild Charge just misses out on a 2HKO.
Worthy of note about this set, it's a lot better outside of Sun.
move 1: Agility
move 2: Hydro Pump / Surf
move 3: Ice Beam
move 4: Grass Knot
item: Life Orb / Air Balloon
evs: 16 HP / 252 SpA / 240 Spe
This Agility set is copied straight out of the analysis (written by ParaChomp). In OU this really just doesn't quite pack enough punch to be worth it most of the time, especially now with Porygon2 to ruin your day. In UU, however, if Chansey can be eliminated ahead of time, Empoleon can make a strong cleaner late-game. Coverage is fairly standard but still good, hitting lots of common Pokemon for SE damage. STAB Hydro Pump is nothing to scoff at either, coming off of 111 base SpA.
Counters: Chansey. Even though Empoleon is immune to Toxic, it's walled absolutely by Chansey and can't break through. The situation is similar with Sigilyph after one or two boosts, but Sigilyph has the advantage of easily being able to put a burn onto Empoleon.