Here is the full list of all 20 Leviathan: Warships achievements.
Finish Mission 3 of The Arctic Serpents campaign.
Create a fleet.
Create a blueprint.
Finish a multiplayer match with four Players.
Add a friend to your friend list.
Complete the 'The Sinking of Charybdia' Challenge.
Complete the 'Defense of Outpost 77' Challenge.
Complete the 'Skirmish in the Floral Isles' Challenge.
Complete the 'Attack on the Strait of Labris' Challenge.
Complete 'The Arctic Serpents' Mission 2 without losing any platforms.
Complete 'The Arctic Serpents' Mission 5 without destroying your pursuer.
Complete the 'Battle of Triem' Challenge.
Complete the 'The Arctic Serpents' Campaign
Win a 4-player game where everyone is fielding the Atlas class exclusively.
Win a 4-player game where everyone is fielding the Buzzer class exclusively.
Win a match without losing a ship.
Hit three enemies with a single railgun shot.
Hit five enemies with a single railgun shot.
Finish the tutorial.
Finish Mission 6 of The Arctic Serpents campaign.