To learn the strongest skills for a Twilight Circle you have to go to specific mental trainers who can improve your mental skills for the respective circle. You do not need to max out all the previous skills on the tree to be able to unlock the strongest skill. Raise a skill one level to unlock the next skill, the strongest skill is the last on the tree. Here are the trainers and the last skills for the specific weapons you need to learn to unlock the corresponding Master achievement:

First Twilight Circle - Raven's Eye (Nox - Outer City)
Second Twilight Circle - Blood Vortex (Edmundo - Water Gateways)
Third Twilight Circle - Scream of Death (Amago - Arsenal Catacombs, a skeleton you can only see in the Twilight World)
Fourth Twilight Circle - Mental Defiance (Amago - Arsenal Catacombs)
Fifth Twilight Circle - Raise the Fallen (Amago - Arsenal Catacombs)
Sixth Twilight Circle - The Final Judgement (Amago - Arsenal Catacombs)