Mages can be pretty powerful solo, at least once you are 25+ and get some spells. And once legendary... there are entire high zones you can just destroy.
The main thing you need to do, as Sart pointed out, is to not take on more than you can handle and to stick to zones/mobs you can fairly easily kill without much risk to yourself. Sure, that 90k kill is enticing, but its normally better to blast through 20-40k kills that are little risk but that you can kill 3-5 of em a tic.
The key to soloing for xp is to minimize the risk while ensuring a steady collection of xp.