Here is the full list of all 22 Lil Tanks achievements.
Complete Stage 3
Unlock Colonel Faulk's Tank
Complete Stage 2
Complete a stage destroying every enemy
Reach Level 4
Complete Stage 1
Use every Cannon Upgrade at least once in a single stage
Unlock your first Hardcore Mod
Complete a stage without a Cannon Upgrade
Complete Stage 4
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
Complete a stage with a Hardcore Mod equipped
Defeat every Boss in Boss Training Mode
Complete The Underwater Stage
Unlock Cuur Corrupted Tank
Unlock General Nero's Tank