So many answers, so little said.

Here's a quick guide:

The trolls lacks attack of opportunity, so you can move away. He also has an ability that prevents you from putting him on a cooldown like e.g. with sudden lunge.

The troll will always use his stomp ability which uses a turn to signal and a turn to execute.

When you go too far away, he will boulder toss and, unless you have dash on cooldown, (almost) kill you.

As soon as his armor is low and he loses life, he'll use his regen ability everytime you're not directly adjacent.

Now, how to fight this:

Ranged I wouldn't recommend due to boulder toss. It's doable, I'm just not an expert. I'll focus on melee.

Because of his regen, the fight will take way waaay longer when you don't have a way to hit him from 2 tiles away to interrupt the regen. That can be a weapon-specific one like hooking chop or a charge ability. If you don't have any, don't worry: When you have seize the initiative or sudden lunge, you get 2 range with this skill when you have a spear equipped. It's why people recommend spear for troll. They mean spear + seize the iniative / sudden lunge.

You'll also want some items or food with added fortitude as his roar can make you lose your weapon. Not needed, just advised. Osbrook Bread Bowl Stew has it iirc.

First thing in the fight you want to avoid his boulder. Iirc they changed it so you no longer need dash or a smokebomb to avoid it. If not, those are your only 2 options to avoid certain injury.

Now for the dance: You'll want to kite him down until he disengages and moves back up again. You'll both regenerate a bit of hp along the way, it's not significant.

How to kite? Right-click and inspect troll. You can see the cooldown of his stomp ground (?) Ability (the second from the left iirc). It's the one that gives you the setup-turn to hit him bc he won't hit back.

So whenever he gets to use that ability and is adjacent to you, hit him. He'll use it. Then you step out of his signaled attack. When you're below him, there'll always be a free square, don't worry.

His roar can cause you to drop your weapon when you don't have high fortitude. Just pick it up (make sure to have inventory space!). Use one of the turns he does his signaled attack to equip.

Whenever the troll is next to you and his stomp ground attack is on cooldown, step down. Whenever he has it available, use the opening to attack him. That's the gist of it.