You know those cage things with an animal head looking like a lion?
Since the final boss could be a lion who's the leader of the Beast Pack...
Here's a couple of ideas for what the fight could be like:
(In my imagination)
The lion leader mutates into something like a fusion between the Chimera and Manticore of Greek Mythology, both being lion monsters with one having wings and scorpion stinger and the other with a goat and snake tail.
Since Elfilin's name reminds me of the word 'elephant', I can picture him turning into a giant pachyderm for Kirby to ride on like a mount to take on the mutated lion leader.
Possible attacks for Elfilin as a elephant includes charging, using his tusks, a trunk to grab stuff and a stomp attack.
For the lion leader, he could use his claws, wings to fly around, the goat head to headbutt and the snake tail's mouth opening up revealing scorpion stingers instead of fangs to bite.
The location of said fight could take place in the lion's lair/fortress where the finale takes place with said animal battle in 3D.
Not much, but would be pretty awesome!