Kill the Wormface boss

To complete this square you need to defeat Wormface, the field boss in Altus Plateau. Wait until the "ENEMY DEFEATED" or equivalent screen appears to mark the square.

This is a field boss that does not require any other boss to be defeated, so it can be done as early as you get access to Altus Plateau.

Wormface counts as an Altus Plateau/Mt. Gelmir boss.