Each class gives you a Heroic Skin once you hit level 20. To get this achievement just level a class to level 20. The Heroic Skins are Red rarity.
Currently the best way of leveling up your characters is to do the Featured escape on the highest difficulty that you can do it on, making sure that you can get Gold. This will also net you:
The Solid Gold achievement in Gears 5 worth 44 pointsBeat the Gold Time in a Featured Escape If you want to know which Heroic skin comes with each class here is a list:
Blademaster - Heroic Venom Claw
Demolitions - Heroic Venom Lancer GL
Infiltrator - Heroic Venom Gnasher
Marksman - Heroic Venom Longshot
Nomad - Heroic Venom Markza
Anchor - Heroic Venom Boltok
Brawler - Heroic Venom Scorcher
Gunner - Heroic Venom Torque Bow
Pilot - Heroic Venom Dropshot
Veteran - Heroic Venom Retro Lancer
Combat Medic - Heroic Venom Lancer
Jack - Heroic Jack / Heroic Venom Hammerburst
Mechanic - Heroic Venom Enforcer
Robotics Expert - Heroic Venom Embar
Tactician - Heroic Venom Boomshot
Architect - Heroic Venom Snub
Protector - Heroic Venom Talon
Slugger - Heroic Venom Overkill
Striker - Heroic Venom Breaker Mace