Your party looks very melee-heavy. With such parties, i often don't use bows or slings (after the start). Instead, i use both short and extended melee weapons. In this case, Mook for samurai is an excellent choice. And the stamina drain isn't that much a problem then, since he'll only use giant sword if no enemies are in close range.
With this party, it would be:
Samurai - Marumasa + Enchanted Wakazashi / Giant Sword
Monk - Hand & Feet / Zaitochi Bo
Valkyrie - Mauler + Diamond Eyes / Dread Spear
Ranger - Bow or XBow only
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