I farmed Arbor with a 55hp mesmer, using Empathy, and Backfire. Run to the lake, avoiding aggro along the way and cast Protective Spirit. Once at the lake, wardens will jump out of the water, break a left and run along the water. Any chasing mob will quickly break aggro. At a point (near a slope that allows you to leave the water) Arbor will engage you so you don't need to pull him. He attacks you, you kill him.
To speed things up, I later swapped Healing Breeze for Shatter Enchantment as I found HB not necessary for the run.
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