I use my alt 85 lock to farm MC for certain materials. I found affliction has too little burst to down low hp enemies quickly, but demonology is amazing. Blow all your cooldowns often and you can cut through anything, plus the felguard will never really die if you talent Fel Synergy. Soloed ragnaros effortlessly on my fresh 85 so it must work pretty well for other instances. Fel bladestorm will probably burst through most trash packs alone.