With my four lvl 42 shaman I was able to burn off 30% of a lvl66's warriors health after he started attacking my group. If I had seen him coming he would have lost a lot more.

It is far easier to kill players far higher then you then it is to kill MOB/NPC's. If a player, though lvl 70, has crappy greens, little or no resists and is the wrong talent spec the probability of burning him down goes way up.