Here is the full list of all 28 MeltDown achievements.
Collect 5000 Coins.
Finish tutorial.
Upgrade a weapon to the maximum level.
Complete a mission without losing any health.
Obtain a 3 star rank on a mission.
Destroy 1000 bots.
Finish an online game.
Finish an online game with 4 players.
Complete a mission in less than 2 minutes.
Destroy 30 bots with a melee weapon.
Destroy 30 bots with the Flamer.
Complete 10 online games.
Pickup 10 Nukes.
Destroy 30 bots with the Shotgun.
Melee Kill a Fuel Bot.
Destroy The Barrager
Own all weapons.
Max out all skills.
Destroy an enemy of each type.
Destroy 30 bots with the Shocker.
Collect 100 Chips.
Reach 100% Critical Strike with the Fury skill.
Destroy 30 bots with the Grenade or Rocket Launcher.
Do a Multi-Kill.
Get Level 1 Prestige
Get Level 3 Prestige
Destroy The Enforcer
Destroy The Electrocutioner