Death by cliff racers. Cliff racers are pretty much always faster than you, so you'll have to kill them extra quick.
*edit*
FWIW, I think your best bet will be a build centered around Destruction. I think I say this because I play with a magicka regen mod, but basically you can train destruction without having to fight anything, unlike Marksman...
...Not that I find sitting around casting spells for two hours to be fun. Remember: You're supposed to have fun THEN complete the challenge, not the other way around