Before Trevenant got booted to UU, I enjoyed using this set. I still get a kick out of using it in Battle Spot sometimes:
-Substitute
-Leech Seed
-Will-o-Wisp
-Phantom Force
Nature: Impish
EVs: 252 HP/180 Def/76 Spe
Item: Sitrus Berry
Ability: Harvest
My favorite is probably Protect, Scald, Toxic, Wish on Vaporeon holding a Leftovers, though I also really like Thunder Wave, Air Slash, Roost, Aura Sphere/Protect on Togekiss holding a Razor Fang :D
I'm don't think that there's any combination of moves that's super effective against all 18 types. You can get close, like with Electivire's Fighting/Ice/Electric/Fire coverage that Moxie mentioned.
But that's why Pokemon battling is a team thing and not an individual thing. As good as any Pokemon is, no one single Pokemon can cover everything/do everything (generally speaking). If there's a type your Pokemon has no way of handling, have a teammate deal with it.
Nah ンン
“No, I... I have to be strong. Everyone expects me to."