A little faster method:

1. Set your unit filters in whatever way you like. Pick a one-slotted girl, so you only have to re-equip one gear. My filters for this purpose is Speed High-Low and Banana Island (as that's my favorite nation). My Primrose has one slot and is near the Unequip All button, so I'm using her for this trick.
2. Hit "Unequip All" above the selected unit's slots.
3. Add equipment.
4. Repeat Steps 2 and 3 as needed till a good number of raid bosses pop out.