As an additional tip, you could probably spend hours spinning the gem gacha for them +1 eidolons and weapons to completely max out your weapon and eidolon grid. Gems taken from grinding event and Sunday SP (which happens to be good for grinding rank too) are basically your main source here. This way you gain a total of +2970 extra atk from the weapon grid and +1782 atk from your eidolon grid.

Of course, everything being said that's a time-consuming task and you should probably ignore this comment and grind other better sources.