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 FoxSpirit
Summon Demon
Summon Lamia
Collect all kinds of units
Summon Arlaune
Summon Scylla
Summon Centaur
Summon Slime
Earn a total of one million gold
Summon Harpy
Summon Kobold