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