His armory A. You have WAY too much hit. You are over the hit cap just with gear, but then take talents boosting it as well. You need 17% hit from gear+talents. Right now your Arcane spell hit is at 24.36%. Move your points from Arcane Focus to knockback resistance, and try and drop some hit from gear. Most specifically, get rid of that +hit trinket. Grabbing the Talisman of Resurgence for Triumphs will be an okay upgrade. Better on-use effect, and you get Int rather than Hit (which gives you longevity, SP and crit).
B. You are using SP/Stam gems. Don't. Use SP/Spirit in blue sockets and SP/Haste in Orange ones. Use pure SP in red ones. In your Belt of Omission, use the Nightmare tear in the Blue socket and then use two pure SP gems. For a more comprehensive report, check run yourself through Rawr.
C. You take enchants aiming more at survivability than at DpS. Instead of SP/Stam, go for SP/Spirit on your legs and SP/Crit on your shoulders. Take Black Magic on your weapon. Are you using Flasks for fights? If not your Alchemy is going to waste. Are you using food? Eat SP or Haste ones.
D. How is your mana efficiency? Are you and your friend both evocating the same number of times? If not, that's a huge difference--8 seconds of no damage is a long time. You didn't max out Arcane Meditation--do so if you have any mana issues.
E. You aren't using Mana Shield are you? I guess specific fights may work, but it is a HUGE mana drain. I advise not unless you have to. That, of course, makes Arcane Absorption useless for most fights. Use it's points for the Spirit Buff/Mana Regen buff. Move Magic Attunement to the other one.
F. What, exactly, is your rotation. And what is the actual difference in gear level? Is he also 2pc T10? I certainly hope he doesn't have 4pc T10 (and then he would pretty much outgear you, even if his average ilvl/GS was lower).
G. How do you use your CDs?
But seriously, you should download Rawr and consult it. Just google it.
[EDIT]
Moz, I think he was referring to Spell Power, not DpS. Otherwise there is a far larger issue here than gear.
Edited, Apr 27th 2010 8:20pm by idiggory