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