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

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...