From what I've read/experienced:
INT increases combo damage
MATK increases base damage
Magic Accuracy of about 460~ is enough to not get resisted most of the time.
For those unable to spend a lot on gear, prioritise MACC and INT above MATK. I'd recommend Electrum Scepter + Shield (one with macc on) especially if you're Twin Adder (Gridanian Shield) as these are 2 of the best pieces gear outside of melds for MACC. AF is fine but if you have the gil for single melds I'd recommend upgading the hat and legs first - Felt Hat of Intelligence and Woolen Tights are my personal choices. For the hat you can meld either INT or MACC, and the legs you can meld INT and MND or INT and PIE - whichever you choose is up to you.
From then on I'd use your jewlery slots to boost your MACC to the required level then focus on INT - filling in the slots with MATK when there are no alternatives. Your belt is best used for an emnity- meld but those are expensive so you may not be able to afford this.
