You get points for killing enemies- so, prioritize clearing mobs, but avoid the named bosses like Lapilla if they're giving you trouble and causing you to lose a large number of Withered. After you've killed enough enemies to reach 400 points, backtrack and then send Withered back with chests. If you send Withered back with chests as you find them, you'll have to continue clearing with a smaller army, since you lose 5-10 Withered per chest.
Go for the Treasure Chests first that only need 5 withered. These usually have buffs for your army in them- 25% increased damage, health, and lower chance to run when at low heath. Very useful.
You get send out either when you die, or when you have no remaining Withered.