WEC and SMF(if you can lock the mobs there properly) Would be the best place to train your 60 masters. Have to be Lv63 to enter both places iirc, although its recommended you be 66-68 when you actually grind your masters there, you'll have higher stats, and stronger Archmage skills
I suggest doing your story quests in Prism Island and Twin Lion mountain and the various reps there to gain Four Seas Fame while you level up to 63(plus, pretty sure a book at Twin Lion gives an Insight Stone, and one of the Four Seas Missions gives one)
If you intend to cap 60 though, you can still use WCM, while it gives reduced exp, Its still better than mobs in other areas(gave my 60 Sage around 20k exp per kill with lega) or if you dont mind spending a lot of money, you can try playing Oclay for Insight stones on Round 5(1 free Mastery point per stone)