Here is the full list of all 16 Monsters achievements.
Collect 50 Power-Ups!
Get a combo of 250!
Get a combo of 100!
Collect 10 Power-Ups!
Collected 75 Soul Hearts!
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Killed 100 Monsters!
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Killed 1000 Monsters!
Killed 10,000 Monsters!
Collect 750 Soul Hearts!
Collect 1,700 Soul Hearts!
Collect 7,500 Soul Hearts!
Collect 100 Power-Ups!
Collect 500 Power-Ups!
Get a combo of 500!