Quick Guide: For Indomitable, all you've got to do is hunt 50 Tempered Monsters.
Tempered Monsters have a purple colour on their names and their scoutflies are blue. So you can tell between normal monsters and tempered ones.
The higher the level of your scoutflies is, the faster you'll find their tracks.
The easiest way I myself found was joining an SOS Flare and searching for high level quests (level 9)
Optional Quests and Investigations are your best choice for this achievement.
Just be patient, once it takes some time until you get it done.
Once you've hunted 50 tempered monsters, the achievement will be yours.
Hope it may be helpful!