Sphinx has very high base atk, 2nd highest of all the event eidolons and even higher than some max gacha eidolons. Unless you're using a full roster of even stronger max break'd gacha eidolons, you should probably at least max her out.
Ah yes, I will max her out, however those weapons don't motivate me to focus almost of my schedule on the event. At least waiting for AP regen should suffice, and then suit myself on what are the left overs once I MLB'd the eidolon.