You'll unlock a Nekko mount at the end of Mission 2 in Koboh, on your way back to the Mantis you'll need to tame one to escape (story related). Soon as you have him tamed find the edge of the map or a sheer cliff. Walk off the edge while riding the mount, then press

to jump off, landing Cal somewhere safe. But leaving the poor mount to plummet to his presumed death.
Long as the mount doesn't respawn and you also land safely (without respawning) the achievement should pop.