After the pirate, where you first encounter the sledgehammer enemy, it is possible to shift to a person at the top right of the room and access a chest next to him. It says "You have found a secret" but I did not get the achievement.

The Sewers entrance level, in the small arena right after the Rat boss, first encounter with big explosive guy (not the kamikaze), bottom right, behind the wooden crates

引用自 Soberian：

The Sewers entrance level, in the small arena right after the Rat boss, first encounter with big explosive guy (not the kamikaze), bottom right, behind the wooden crates

THANK YOU! Not being able to find that crate was driving me crazy