I encountered one on the way to High Hrathgar, not sure what level I was but not too much higher than 10, I don't think. The troll one-shot Lydia, but she gets back up after a little bit, so that will distract it for a while. You can run faster than the troll, so I got a shot or two off as it was approaching, then ran past it as it started to attack. This is somewhat difficult to time right, especially in enclosed spaces, and it takes a long time, but it can be done, but only if you do more damage over time than its health regenerates, obviously. (Poisons help for that.) Fortunately for me I accidentally led the troll into one of the pilgrims at a shrine, who started attacking it. The troll focused on her for a few seconds and our combined damage was enough to kill it.
There's actually an in-game book on this topic. As it says trolls have a weakness to fire, but that's not too helpful if you're not a destruction magic user. As far as I know there are no flaming arrows, but you can enchant your bow or other weapon to deal extra fire damage.
For those who wield 1handers with shields, equip a torch, then "shield bash" the troll with the flaming torch. You will roast him a bit and have an easier time chopping him up with your 1hander. Milkdrinkers can re-equip their shields for better blocking, and true sons of Skyrim can just use their torch for the whole fight.