It depends on the leve and the location and you, really. Some leves are just harder than others. I'm sure there will be some folks popping by to tell you that they solo'd all their r40's easily at r34. But as for me, I found that I could solo most r40 leve mobs around r37-38 (but not anything with mongrels or zombies, those are really hard!) as a mage.
In my experience, if you are doing your leves solo at a low-ish rank, the camp you choose makes a big difference. Camp Broken Water may have the "best" r40 leves, but they mostly involve mongrels (see above, lol), not to mention that for many leves you have to deal with aggro mobs galore which can make it really hard. Camp Nine Ivies and Treespeak have slightly less aggro to deal with, and IMHO Camp Bald Knoll and Halatali were the "safest" spots to do R40's at. Camp Iron Lake is probably low aggro too but I didn't do many leves there so I'm not sure.
Though are you sure you don't mean Camp Nine Ivies? 'Cause I don't recall aggro being an issue on the way to Treespeak, assuming you're coming from the direction of Camp Emerald Moss.