People talked about this set a while ago, but no one actually posted it.
Jolteon
Magnet Rise Passer
Substitute
Magnet Rise
Baton Pass
Thunderbolt / Charge Beam / Agility
Item: Leftovers
Ability: Volt Absorb
Nature: Timid
EVs: 6 HP / 252 SpA / 252 Spe
Simply come in on something that will switch out and set up a Substitute like usual. If they switch to something with a Ground move, use Magnet Rise. (Even if they use a Choice Scarf to outspeed you, the Substitute will protect you and you'll end up comepletely walling them.) Then you can freely pass your Magnet Rise and Substitute to something like Heatran or Metagross and eliminate one of their weaknesses.
Thunderbolt is Jolteon's main STAB but you can use Charge Beam if you want to try and pass a SpA boost. Agility might seem pointless on something as fast as Jolteon, but if everything goes as planned you'll get a free turn and many Pokemon can be deadly with that much speed. If you decide to use Agility, move the SpA EV's into HP.
Darkrai
Nasty Plot Sweeper
Nasty Plot
Dark Void
Dark Pulse
Focus Blast
Item: Life Orb
Ability: Bad Dreams
Nature: Timid
EV's: 6 HP / 252 SpA / 252 Spe
The idea is to Nasty Plot first as a Sleep Talker comes in. Darkrai can OHKO all Uber Sleep Talkers except Kyorgre, Groudon, Heracross, and any Arceus it can't hit super effective. If you end up facing Giratina or Dialga, however, you can simply Dark Pulse or Focus Blast it respectively. Even if you end up using the wrong move on a switch-in, you can still usually KO it with your other move. Here are some damage calculations. (They're probably a few points higher because I don't think the damage calculator I used constantly rounds down.)
0/252 Calm Blissey
Dark Pulse: 31.6% - 37.3%
Focus Blast: 63.3% - 74.3%
Total (with Leftovers): 88.7% - 105.4%
32.3% chance to KO, guaranteed with SR
252/32 Modest Fighting Arceus
Dark Pulse: 32.9% - 38.5%
Focus Blast: 65.8% - 77.5%
Total: 98.7% - 116%
92.5% chance to KO, guaranteed with SR
252/32 Modest Bug Arceus
(same as Fighting Arceus but with Dark Pulse and Focus Blast switched)
248/252 Calm/Careful Ho-oh
Focus Blast: 22.4% - 26.5%
Dark Pulse: 45.1% - 52.8%
Total (with Leftovers): 67.5% - 79.3%
KO only with SR
0/6 Adamant Ho-oh
Focus Blast: 34.0% - 39.9%
Dark Pulse: 68.3% - 80.5%
Total: 102% - 120.4%
252/176 Careful Deoxys-D
Dark Pulse: 125% - 147%
Choice Scarf Heracross still completely screws up this set, but it screws up most Darkrai anyway.