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