Here is the full list of all 14 TINY METAL achievements.
Heal an Rifleman to full health at a city
Win any mission
Destroy a tank with an Rifleman's attack
Rank up any unit to Rank 5
Use Focus Fire to achieve a 100% or higher single attack on any unit
Complete the single player campaign on New Game+ difficulty (Act I ~ IV)
Complete MX04 mission on normal difficulty
Complete MX04 mission on New Game+ difficulty
Win a mission with only 1 surviving unit
Win a mission with 50 or more surviving units
Rank up any unit to Rank 13
Rank up a Rifleman to Sergeant
Rank up the Danté Carver hero riding the MEGA METAL to rank of Captain
Complete the single player campaign on normal difficulty (Act I ~ IV)