best way to kill a troll quick is to parry it and give it a hit or two with the parry damage multiplier, then repeat the process. It is stunned for quite a while to eat a number of hits each time. Get the timing right, this game is super forgiving in it's parry window so just have your teammate maybe stay behind you and shoot arrows at it, or get back, so the troll doesn't switch targets and you parry his every hit and give it a huge chunk of damage. You slaughter it like a boss without even moving from your spot