Here is the full list of all 15 Johann Weiss achievements.
Complete both sewer levels in Episodes 1 and 2 in one single playthrough
Defeat the Cult Leader in Episode 1.
Defeat Mr. Skull in Episode 2.
Defeat the Hellcube in Episode 3.
Defeat Behemoth LaVey in Episode 4.
Kill a Banshee
Kill a Golgothan
Find and pick up Dopefish
Die 10 times in a single playthrough
End a game with a score of over 1,000,000 points
Finish a level with a time bonus
Open the Help Me section in the main menu
Kill 10 Golgothans in one single playthrough
Kill 20 Medusa heads in one single playthrough
Jump on top of Mr. Skull's head