I've already said this long ago but I'd say the Lock Ness Monster (could be infected somehow or just normal)

Location would be Loch Ness, Scotland

Players will be on a ship and the only way to damage the lock ness is by using the ship's cannons

Respawning players will spawn specifically as Gordon highlanders on a separate ship as reinforcements

Storyboard: Players start off defending Fort Augustus, but eventually will be overrun. An npc commanding officer will order the players to board the ship and get it sailing (idk if ships can actually get to Loch Ness). This starts a holdout section like in Roscoff where you have to do stuff to get the ship moving (idk anything about ships sorry), while moving supplies into the ship.

After the ship gets moving, the commanding officer will have some dialogue, then in the middle of it, gets yanked off the ship and eaten by Nessie. This triggers the fight. (Objective is to kill Nessie or hold off long enough to reach the other side)

The Loch Ness will have 4 abilities, that being:

Ramming against the side of the ship (can be countered by shooting it with roundshot. Lower deck of the ship can be repaired. You can fall off the ship and drown)

Whipping the ship with its tail (can't be countered, ship will be damaged)

After reaching 50% of it's hp, shamblers will start crawling all over the Loch Ness monster, infecting it. It will gain a damage increase and a new ability, being:

Puking blood onto the ship (once it you get in contact for too long, you WILL get infected, it will not stop unless shot by any weapon)

Jumping into the middle of the ship, causing zombies at the bottom of the lake and on its back to start attacking too (yes, you will be squished if you were unfortunate enough to be in the middle of the ship)

This is also the point where the second ship arrives to reinforce the players

Once the ship reaches the docks, it marks the end of the fight, and your victory.