Deviant Gatestones on RSBuddy.
You need 125 cosmics binded to you in dungeons, access to complexity 3 (just do 2 dungeons in dungeoneering), also you need 32 mage to start.
So first use AlternativeMagic on RSBuddy to curse Zombies in the Varrock sewers. You'll get 32 mage within an hour, then move on to Deviant Gatestones.
With Deviant Gatestones, you'll get like 50K-60K XP/hr, and you'll have 75 magic in like 20-25 hours.