eldar seem pretty good at killing mephiston. runes to prevent his spells, guide for your 3 warwalkers with starcannons is nice. 4+ to wound, no armor save for him. throw in a doom if ya can. fragons and wraithguard if he ends within shooting range. hehe charge him with 10 banshees and pray for 6s to wound!

as far as spacewolves go, you can JoWW and hope for the best. load up on power weapons/fists.

i imagine it would be expensive, but a thunderwolf lord with mark of the bear (EW), a storm shield and frostblade (for 3+ invuln and 4+ to wound) would give him a good fight. givem a wolf necklace so he always hits on 3s. thats 5 attacks on the charge/countercharge, unsaveable wounds. keep a runepriest nearby to dispel his spells