Grunthors spawn on Sargasso. You can either take your time exploring every time you visit or you can pick one of the Grunthors you fight during your first visit and make it respawn by dying to the acid right after you kill it.
1. Explore the planet instead of following the bot to the Morts.
2 and 3. Before and after the story-related bridge.
4. During Trudi's quest Rivet will make a comment about a "big fella" and there's one in that area.
5. Late in the story, you come back and there will be one.
6. Skeletal Grunthor during one of the Bronze cup battles in Zurkie's.