You need all seven Lieutenants to unlock the master key. The master key unlocks the last level (Level 31 – Mars) as well as the last helicopter (Red Rage). These two item are tied to the following four achievements: Alive, Still Alive, Aaaand Still Alive, and Scrap Them All achievements. So unlocking all the Lieutenants are necessary for completing the game.
Before we start there are three tips:
1. The Lieutenants have more health and damage than normal mobs so they may be difficult to defeat on the first try. You can always come back after you unlock and upgrade more helicopters and gunners.
2. You must winch the Lieutenant after defeating them and bring them back to the indicated helipad on the screen.
3. Once you drop the Lieutenant onto the helipad and get the notification from the Alman-O-Tronic, you can simply quit the level and return to the map. You do not have to complete the level.
Lieutenant 1 – Kobag – Mission 6 (Destory the zombie lab)
The first Lieutenant, Kobag, is located in Mission 6 near the start of the level.
Lieutenant 2 – Bardaz – Mission 8 (Rescue 4 survivors)
The second Lieutenant, Bardaz, is located in Mission 8 near the right side of the train with the giant leg.
Lieutenant 3 – Boulderbane – Mission 12 (Deliver food and weapons to the camp)
The third Lieutenant, Boulderbane, is located in Mission 12 near the giant spider.
Lieutenant 4 – Scourgesurge – Mission 14 (Search and Rescue cpl. J. Doe)
The fourth Lieutenant, Scourgesurge, is located in Mission 14 after rescuing cpl. J. Doe. Scourgesurge is wearing a fireman’s outfit and is wielding flame throwers.
Lieutenant 5 – Darkdrooler – Mission 16 (Rescue 6 survivors)
The fifth Lieutenant, Darkdrooler, is located in Mission 16 right near the 1st survivor at the start of the map. Darkdrooler has an army uniform and is wielding rocket launchers.
Lieutenant 6 – Doomlimb – Mission 21 (Defend the convoy from zombies)
The sixth Lieutenant, Doomlimb, is located in Mission 21 after the second save point. Doomlimb looks like a hot cheeto puff.
Lieutenant 7 – Chanzud – Mission 28 (Destroy the tower and build a jet)
The seventh and final Lieutenant, Chanzud, is located in Mission 28 near the forest near the start of the map. Chanzud looks like Doomlimb but with bombs strapped to his body.