It is killable now and latest patch notes as of April, 26th 2017 include-
Combat patterns for Mutant Troll and Mutant Ogre have been changed.
If HP falls below a certain level, they will attack more fiercely.
Mutant Troll’s location has slightly changed.
All players who have inflicted damage on Mutant Troll and Mutant Ogre will now be able to receive loot.

This Boss drops 1x Shattered Ring, 1x Ogre Ring or 1x Soiled Ogre Ring.

It depends how lucky you are, if you are really lucky you can obtain all 3 items altogether.

how do you get the knowledge to S?

The Guide on Mutant Troll https://bdocodex.com/cn/theme/4785/ is exactly the same thing you need to do for proc on Mutant Ogre Knowledge as the quest it is locked behind gives both simultaneously, HIGHLY ADVISE READING IT as not to regret, this knowledge only coming once as far as I'm aware.

Сейчас его можно убить, нас было 6 человек, атака примерно 320-340+, убили за 3 минуты. В соло убила за 30 минут.