I'm heading to the Final Battle against a Lich and his army tomorrow. Need advice. Our lvl 14 party is reached the final stage of the campaign that we have played for two years every week.

I'm lvl 14 Bladesinger Hig Elf with pretty full spellbook, Staff of Power and Ring of Spell Storing.

My companions are Assassin/Gloomstalker, Inquisitive rogue, Valor Bard and Fighter.

We have four months of downtime before the Lich attacks the empire capital with his army, and if we survive, we'll have to destroy it's phylactery under a mountain. So at least two battles, with hopefully a long rest in between.

Which spells should I prepare, and what sort of tactics could work?