This is pretty common knowledge, yet nobody seems to do it.
In patch 4.0.1+ all classes have gotten massive self-healing abilities and talents, which makes them pretty damn OP @ their level with all that Bound to Account gear.
Considering these things, one is able to solo even the most hardcore elites.
- Here's the trick:
Solo instances!
I was able to gain 5 (yes, five) levels in a single Ragefire Chasm run at level 13
Also, I was able to gain 2½ level in a single Zul'Farrak run at level 44
- Here's the trick no more
Post script: with the massive self-healing all classes have received, there's absolutely no downtime at all. One could probably get from 1-30 on one day, then 30-50 the next using this common knowledge trick.