While in town, you can open the menu to change your equipment, use items or save by right-clicking.
To set your passive skill, click on the field in the status menu.
Casting a Thunderbolt during the same or next turn after casting Fayadaan (Luna’s water spell) will cause additional damage to all enemies.
Nonne Cura (doubled healing) also works on items, though not (AFAIK) on Ceremonial Spear.
The Feather Anklet prevents you from activating floor portals, which can help you access certain chests. If you’re trying to use a portal and can’t, take it off.
Octuple Assault requires some combination of MP buffs, a teammate feeding Luna with Mana Drops, and/or Spirit Pill (zero MP consumption) to be effective. It does incredible damage near the end of the chain, though; at level 25 the eighth hit can do over 2k damage.