Once you hit level 100 you can quest for your masters skills!
Pro tip: Before you start, i suggest going to water town and farming the mutated fish for some fish meat hp buffs. This will allow you to tank/kite these monsters right at level 100.
You can find your Masters class trainer npc at any of the class towns through the crystal located near yuba. The map above shows the begining of the quest aswell as all 4 boss spawns.
When looking directly at the 3 pathways you can find the monsters as the picture above illustrates.
The stone golem is found directly across the arena from the kentarus.
Return to quest start and push L to see your new skills.
Dia (masters books merchant) can be found on the shore by blue cliff ahown in the picture below.