Stacking AP = compound AP onto everything. Using elemental attribute, the only stat you should care about really is AP. And you can honestly use the GLS well into the 200s (I've seen a lot of people use it 250+), basically and Ult 3s shield will do, compound HP for a better IR.
Or whatever you see fit. Once you hit like level 180+ you should really be more attuning your gear for what YOU need.