Wait until Arcano starts attacking you and use the Staff of Magnus on the Eye of Magnus until it loses it’s glow and becomes a “solid” sphere.
After that, just kill Arcano any way you want =).
Tip: You might wanna summon an Atronach or something to distract Arcano while you’re still working on the big sphere (sometimes those weird Magic Anomalies do that for you).
I haven’t posted any StarCraft stuff this week, and one of the reasons for that is that I was addicted to this new RPG game – Skyrim. I’ve just finished the game, accomplishing like 90% of the quests (I kindda gave up chasing every detail when I got maximum level gear and skills).