I'd say do it all at once in one go as you won't lose out on the wish credits. Once the rebate is over and you only did it one level at a time and wait to get more runes and level again, you would lose potential wish credits which would potentially benefit you in the future, which is the reason u should do it all in one go. Usually I go from lvl 7-10 or you can even go lvl 6-10

You have pretty much 3 choices:

1) Use rebate every time it is. You get a steady boost of power

2) If you play the long game, save 1353 refines to go from lvl7 to lvl10 in a single rebate (this allows you to make the most use of wish credit which is equivalent of free 350 adv refines). This one focuses on upgrading the gear

3) If you play "the longest" game use x500 every time in order to get purification breakthrough pills. This one obviously focuses on pills.

You need up to 201refines to go from 7->8, 501 to go from 8->9 and 1001 to go from 9->10. Now some explanation: 1) is a bad idea, dont do it. The difference between 2 and 3 is (in current event iteration where 800refines used is last milestone) as follows.

4k refines used for simple calculation, since 3x1350= 4000~ and 8x500=4000:

2) Using 3x1353 guarantees you 3x lv10 equipment piece. You also get 3x52= 156refines, 3x 18= 54keys, 3x pill, 3x lv7 mangatama as well as 100% chance to use 3x350 wish credit each time. You dont always need 1353refines to level up, but its better to be safe than sorry. Also since you will hit max rewards in refine rebate if you dont use all 1353, the leftovers will contribute towards your next rebate.

3)Using 8x500 will get you 8x34= 272refines, 8x18= 144 keys, 8x pill. This one however depends heavily on luck to use wish credit.

In order to max out refine feature you need max of 34060 adv refines and 400 pills. Ammount of pills you can get from refine rebate using 2)option is 20, while 3) is 68.

For an average person that wants to maximize their gains 2) is the best choice. It allows you to save 350refines due to wish credit on each piece of gear, which is 10.5k cp if we assume 1 refine is worth 30cp.

#3 option should be used only when you are either aiming for an insane endgame (you will max out refines way earlier than maxing out pills) or a heavy (or even heavier than heavy) spender. For small/moderate spenders maxing out refines will take around 10 years though, hence its only for whales.

However the longer the game goes the easier (and cheaper) it is to get stats. Refines currently are on rather lower spectrum regarding secondary stats, which in time might make option 3 even less viable because those 20x350refines (150-200k cp) could be spent more effectively.

Yep, once you got all level 8s you just focus on getting move 1 level 10s only with swb refinements and by buying them in sun scroll shop. It's since charm introduction that spending coupons on refinements is on the lower side of the spectrum.