Here is the full list of all 34 Viktor achievements.
Complete the game on normal difficulty
Defeat Mephistopheles
Defeat The Frozen
Defeat The Corrupted
Find all of the weapon blueprints
Find the rocket launcher blueprint
Blindly jump off the edge in the Gate Fortress
Defeat The Djinn
Find all of the shield upgrades
Defeat The Shapeshifter
Find the assault rifle blueprint
Find the grenade launcher blueprint
Defeat The Demon
Find the shotgun blueprint
Defeat The Shadow
Find a shield or weapon upgrade
Find the sniper rifle blueprint
Defeat the Hell Spider
Complete the game without upgrading any of the weapons
Kill the Hell Spider boss using the chandelier
Complete the game using only the dual pistols
Defeat Mephistopheles without taking any damage
Defeat the Hell Spider without taking any damage
Defeat The Shadow without taking any damage
Defeat The Frozen without taking any damage
Defeat The Demon without taking any damage
Defeat The Corrupted without taking any damage
Defeat The Djinn without taking any damage
Defeat The Shapeshifter without taking any damage
Complete the game on easy difficulty
Complete the game on hard difficulty
Complete the game on any difficulty in New Game+ mode
Complete the game on hardcore difficulty
Collect all of the mysterious artifacts