It seems like deathchill slows everything down within its radius. I can't tell you if it works on the big bosses like the Titans...as my Theurigist (Spirit) is only on level 10. (Planning on Rogue 2nd Mastery).
Other trees that have slowing effects:
1)Rogue Tree
-Has Envenom Weapon (a WP skill), which has Nightshade (implementing skill)...and it slows down enemies total speed by a % when hitting them. This is dependent on striking them...while enemies only have to be within a vicinity of deathchill. (I'm going to use the Deathchill in combination with Nightshade...if they stack accordingly...then everything around me should be super slow...hopefully even bosses.
2)Storm Tree
-Has Storm Nimbus (a WP skill), which has Heart of Frost (implementing skill)...and it slows down enemies. Sort of like Nightshade, except that Nightshade has damage over a time period...while Heart of Frost is instantaneous damage.
3)Nature Tree
-Has Plague (PA skill), which has Fatigue (implementing skill)...and it slows down enemies...but you have to press another button to use it and then attack with your weapon...which can create for more tedious effort on your part.
That's all for the skill options.
It's your choice. And usually picking that 2nd Mastery is the hardest choice to make in the game...as the mystic can't remove masteries...only skill points of a particular mastery.
So just go with one of the masteries and stay with that 2nd mastery...and be confident in your choice and make the best of it...even if you were weary of your choice.
Planet_Guardian