Here is the full list of all 34 Viktor achievements.
Find the shotgun blueprint
Defeat the Hell Spider
Blindly jump off the edge in the Gate Fortress
Find a shield or weapon upgrade
Find all of the weapon blueprints
Find all of the shield upgrades
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 normal difficulty
Complete the game on hard difficulty
Complete the game on any difficulty in New Game+ mode
Find the assault rifle blueprint
Find the grenade launcher blueprint
Find the rocket launcher blueprint
Find the sniper rifle blueprint
Complete the game on hardcore difficulty
Defeat Mephistopheles
Defeat The Shadow
Defeat The Frozen
Defeat The Demon
Defeat The Corrupted
Defeat The Djinn
Defeat The Shapeshifter
Collect all of the mysterious artifacts