The safest way is to dig a trench around an area. I usually do this when I make a base next to the black forest. Trolls can't cross it and you can shoot them with arrows. Watch out for flying rocks.

You could kite them with your bow through the black forest. They get caught up on terrain and downed trees which will slow them down. They have to walk around obstacles that you can jump over. You may have to switch to melee to get rid of the greys that join in the chase. This one is more tricky.

Use an atgeir's secondary attack to stun them. Spin once, jab once, repeat.

Some people parry them and then strike. Failing to get the timing right can easily mean death.

Sneak attacks help greatly. Trolls are vulnerable to pierce, which means arrows, atgeir or spears.