Requires 10 horseshoes, and you can fill remaining 7 spots for ★1 mercs. With exp booster you can max level ★1 mercs. Info from GuitarRock FWT's YouTube video. ★6 John+10 ★5 Naius+6
Level Up
Requires 9 horseshoes, and you can fill and level other 7 spots for ★1 mercs. It gives 2152 exp points. From what I hear, if you use exp booster you run once and you can max level them. ★6 Octavia+9 ★5 Maya+9
40-10 is mainly for leveling up 1 merc, the last 9th spot. However you can still get normal scrolls, and good amount of golds. Need 8 horseshoes for hard mode. When other 8 mercs are max level, 9th merc will get 6245 exp (781 exp / horseshoe) which is better than 45-10. ★6 Arines+9 ★6 Ebo...
25-10 is a good place to get normal scrolls, and leveling up for several mercs. Need 6 horseshoes for hard mode. Sometimes Angelica fails to land critical, and won't be able to one-shot the boss. This formation is tuned for clearing as fast as possible, but you can replace Zakan, Chalkle, and Eun...