Only level your equipment till level 6, at level 7 it will start consuming advanced refines and lemme tell ya, you need advanced refines waaaay more than medium refines. The reason why you should only go till level 6 equipment for now is because of how the refine rebate works. After you rank up you get a rebate while the rebate is active. If the rebate is over, all that rebate bonus will be gone. So that means if you level up an equipment to level 7 and you get the rebate bonus but you can't make use of it then you'll lose a lot of advanced refines (important and pretty basic long term power-up strategy).