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