Here is the full list of all 28 Mini Metal achievements.
Reach 50,000 total kills
Collect total 100,000 coins
Deal a total of 5,000,000 damage
Collect total 50,000 coins
Reach wave 200
Reach 10,000 total kills
Deal a total of 1,000,000 damage
Collect total 10,000 coins
Reach wave 100
Deal a total of 500,000 damage
Collect total 3,000 coins
Reach wave 50
Deal a total of 100,000 damage
Reach 1,000 total kills
Collect total 1,000 coins
Deal a total of 50,000 damage
Reach 800 total kills
Reach wave 20
Deal a total of 10,000 damage
Collect total 200 coins
Reach 200 total kills
Reach wave 10
Deal a total of 2,000 damage
Reach 100,000 total kills
Reach wave 500
Collect total 500,000 coins
Reach 500,000 total kills
Reach wave 1,000