Achieving level 15 and getting a phone call from Ron Jakowski.
"A heavily guarded industrial plant out in the middle of nowhere? Clearly they're manufacturing more than just cement here, and don't take kindly to surprise guests. Be prepared for multiple ambushes that escalate in severity with each wave of attack."
Standard Survival rules apply.
Many cover opportunities are available but the best "rooftop" vantage points are now out of bounds.
The enemies are Gruppe Sechs with Merryweather air support in later rounds.