An ER movie should be done like Thunderbolt Fantasy. Almost everyone is a puppet, Ranni remains a puppet, but the other puppets all notice that she is.

Radahn is just a full-size human with a puppet horse.

Erdtree? Regular tree and a bucket of gold paint.

Elden Beast? Find a washed up giant jellyfish corpse, jam in some LEDs and… okay maybe not that one.