Here is the full list of all 22 Lil Tanks achievements.
Complete The Underwater Stage
Last 10 minutes in Endless Mode
Reach Level 8
Complete a stage in Co-Op mode
Defeat every Boss in Boss Training Mode
Beat the Campaign
Unlock General Nero's Tank
Complete Stage 5
Complete a stage with a Hardcore Mod equipped
Complete a stage without losing any hearts
Unlock Cuur Corrupted Tank
Complete Stage 4
Complete Stage 3
Use every Cannon Upgrade at least once in a single stage
Unlock Colonel Faulk's Tank
Complete Stage 2
Reach Level 4
Complete Stage 1
Complete a stage without a Cannon Upgrade
Complete a stage destroying every enemy
Unlock your first Hardcore Mod
Unlock all Hardcore Mods