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