The only thing so far I can one hit shot are the rabbits and chickens, and that's basically for fun and there is an achievement for it. Also easy to hit something for 300,000 Damage in One Hit achievement as well.
Red or Yellow Mobs even with down-leveling system do become easier to kill though with better gear.
On my ele and my warrior (20 and 26) with a powerful attack and a crit hit I have actually 1 shot mobs in the starting areas, I think they are like the lower level mobs. You know how the area you are in are lvl 5 but that mob is like 3 so is weaker, but hey I still one shot it