It will surprise few that Blackwing Lair's Chromaggus will be walking away from our weekly Two Bosses Enter
competition as the victor. While Karazhan's Shade of Aran is a powerful caster with access to three schools of magic to best make use of Chromaggus' magical weaknesses, he would still be out of luck whenever Chromaggus wasn't vulnerable to fire, frost, or arcane damage. Combined that with Chromaggus' ability to frenzy and Aran's low damage, and we have our winner.
For the next in our round of boss deathmatches, check back later today for a discussion of Baron Geddon vs. Lady Vashj.