Mountain Trolls move somewhat slowly, and do not regenerate health while frozen.
As with Armored Ogres, the quickest way to dispatch one of these monsters is to convert it with Spore Mushrooms. They make powerful allies if converted or charmed.
The Sorceress's Polymorph Ring can be very effective against all larger enemies, provided the user is prepared for what may result. See Polymorph Ring for further details.
All Trolls regenerate at a far slower rate when converted with spore mushrooms. The same is true temporarily when charmed using the Sceptre of Domination. This intentional change is likely for the purpose of balance - a mountain troll would be almost invincible to foes with its default regeneration rate.
The Mountain Troll is not actually Immune to stun, despite what is stated in-game. This can be confirmed when checking its game files in the Source XML file set or in-game.