dnd 5e – Planning a one shot for my group for one of my player’s birthday

So one of my player’s birthday is coming up soon and it so happens to fall on the day we usually do our sessions. I was planning on doing a one shot based around her Druid character and all my other players agreed so I have most of the encounters and extra little bits included, but I’m at a loss for what to throw at them as the big boss at the end as they cut through most of the monsters/people I throw at them pretty well as a group.

The group consists of:

  • Half-orc Barbarian; Path of the Totem Warrior: Bear
  • Human Monk; Way of the Shadow
  • Halfling; Rogue: Swashbuckler
  • Tiefling; Warlock: Hexblade (Pact of the Blade)
  • Tabaxi; Druid; Circle of the Land (Forest)

As a note
The One-shot is based on a coliseum battle arena, I’m allowing them to level up to 20 for it, so I really want them to have a fun time with the end boss but not kill it too quickly, they are regularly level 14.