How am I supposed to kill the frost troll?

Your level, your equipment, and your skillset all come into play while fighting. It is good to know that Frost trolls are weak against fire and it will stop them from regenerating health for a bit.

In low levels, they can absolutely devastate you, so make sure you are at least level 10 or so before you want to take one on.

Think of it like a deathclaw in Fallout 3: you can find them at level 1, but that doesn't mean you can fight them.

Dragons scale in level relative to you - Frost Trolls do not.

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.

As a mage here is my advice:

Invest in "Destruction Dual Casting" - a level 20 destruction perk.

Dual cast Firebolt - it doesn't use very much magika and your magika recharge should be fairly close to magika use. Each shot will stagger the troll buying you precious seconds and preventing healing. It will take a minute but you can effectively kill a disabled troll - easy!

With high magika and magika regen you shouldn't even need to use any potions. And if you've got a companion they can help you out without taking any damage.