Alpha Mammorest is a level-38 boss around (189, -478), near Grassy Behemoth Hills in Windswept Hills. It is easy to reach from an early travel point, but this is a fight to come back for once your party is ready.
Look east from Grassy Behemoth Hills
From Grassy Behemoth Hills, look east toward the encounter area at (189, -478).
Check the target's identity and level before attacking. Ordinary Mammorest also spawn in the field, so the first one you see may not be the named Alpha. Use the coordinate to find the area, then confirm which Mammorest you are approaching.
If you are just passing through, give it space and be careful about attacking other Mammorest nearby. The species is generally unaggressive until provoked, including when another Mammorest is attacked.
Prepare for Grass and Ground
Mammorest is Grass/Ground in 1.0.4. Account for both elements when choosing your attacks; older Grass-only advice misses part of the matchup.
Check the attacks your party actually has equipped, then use the element guide to plan the matchup. A Pal's own element does not tell you which attack it will use next.
Use a few controlled exchanges to judge the fight. If clean hits barely move Mammorest's health while your party is taking heavy damage, retreat and improve your equipment or levels. More ammunition will not make up for a large progression gap.
Keep a weaker attack for the capture
Mammorest's large health bar can make the opening feel slow. Resist the urge to keep every damage source running once it gets low: several attacks landing together can finish the boss before your sphere reaches it.
Keep a weaker weapon available, and recall your attacking Pal when you need to control the final damage. Check the capture chance before spending your best spheres.
After a catch, Mammorest can help with gathering: its mounted partner effect improves logging and mining, and its riding technology unlocks at level 28. Craft the equipment and try a familiar resource trip to see how it fits your routine.
If the Alpha is absent, check the encounter coordinate and whether another player recently completed it. Searching farther into ordinary Mammorest habitat will not find a replacement for that fixed encounter, and respawn timing can vary with your world.
Browse the Pal location finder for another target. Kingpaca is a lower-level fight, while Chillet is an earlier riding project.