Simply complete all 50 waves of regular horde mode (Any difficulty) on any of these maps and the achievement is yours:
Avalanche
Blood Drive
Canals
Checkout
Clocktower
Dawn
Diner
Drydock
Forge Blitz
Fuel Depot
Glory
Harbor Haze
Hotel
Impact Dark
Lift Apex
Mercy
Old Town
Raven Down
Reclaimed Windflare
Rustlung
Security
Speyer
The Slab
War Machine