Here is the full list of all 12 MONMUSU achievements. It takes around 4-5 hours to unlock all of the achievements on Windows.
Summon 9 monster girls
Summon Demon
Summon FoxSpirit
Summon Lamia
Summon Scylla
Collect all kinds of units
Summon Centaur
Summon Arlaune
Summon Harpy
Earn a total of one million gold
Summon Slime
Summon Kobold