2/?
For the moment, let's assume everyone in the party is level 18 when they go into the fight against Vecna just to keep the options simple (I know they won't be; some will be level 20, others will be 19, and I think Pike will be the only level 18 in the fight).
Level 18 Moon Druid:
24 Prepared spells.
Level 18 War Cleric:
23 Prepared spells, 10 Domain spells.
Level 18 Lore Bard:
24 known spells
Level 13 Assassin Rouge/5 Vengeance Paladin:
Four 1st level, Two 2nd level slots.
Level 14 Ranger/4 Assassin Rogue:
8 spells known, Four 1st level, Three 2nd and 3rd level, One 4th level spell slots
Level 15 Berserker Barbarian/Battlemaster Fighter 3:
3 maneuvers, lots of Rage
Level 18 Gunslinger Fighter:
Lots of trickshot uses and two Action Surges, along with Hex
Pre-battle buffs:
Heroes Feast the night before. Between the Cleric and the Druid, they blow their 4th level slots and a 5th level slot from the Druid on Freedom of Movement on the party.
Bard looks to use a mix of Bardic Inspiration and Cutting Words along with his higher level spell slots on Counterspelling the large stuff from Vecna (Finger of Death, Power Word Kill, Disintegrate, etc.), uses the lower level stuff to keep everyone topped up via Healing Word.
Cleric uses her spells on Cure Wounds, Mass Cure Wounds, Heal, etc., while helping to beat up Delilah if she's on the ground. Otherwise Cleric stays in back and is on defensive to protect Bard and Druid.
Druid looks to provide more healing or use some save or suck spells, or the spells in the staff, to try and burn through Legendary Resists.
Grog, Percy, and the Twins blow up Delilah, looking to take her out quickly before she can overwhelm the combined focus firing that the Bard will draw upon himself. After that, you kill or disable the Death Knight, ideally by removing his weapon and then subjecting him to Hold Person or Banishment.
After that, you blow EVERYTHING on Vecna, overwhelm him with action economy.
