The Trickster Elf spawns on Wave 38 in TDS Frost Mode, where it appears alongside the Frost Hero and Frost Necromancers.

Quick scoop

That wave is the important checkpoint to prepare for, because the Trickster Elf shows up in a late-game spike rather than early waves.

Note

Some older or alternate wave discussions may vary by mode, but the source I found specifically ties Trickster Elf to Wave 38 in Frost Mode.