To unlock the strongest skills for a weapon you have to go to specific combat trainers who can improve your physical skills for the respective weapon. 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:

Scythe - Hurricane (Rangar - Outer City)
Sword - Death Birth (Rangar - Outer City)
Spear - Spear Fury (Akbah - Africa)
Hammer - Bludgeoning (Akbah - Africa)