Credit goes to Tricka
for writing the solution from four years ago:
I decided to give a little more detail below for anyone that needs it.
Start up a new game of Lifeline.
Clear out the first mob of zombies and get to the truck. Should take no more than two minutes.
If this is your first time playing then follow the blue arrow on the mini-map in the lower-left of the screen to the LZ/base. (Press the view/back button
if you need to see a full map of where you're going.)
Once at the destination, run into the gated area which will activate the NPC soldiers inside to fight. You can run over to the supply locker and grab the Grenade Launcher if you want.
on the locker
to get to the gun menu
-Highlight the Grenade Launcher and press
-Then move your cursor to your inventory and highlight the Launcher again and equip it by pressing
to close it
Once all zombies in the area are dead it will go to a little cutscene, you should get your progress right as it starts, feel free to quit out and then start a new game to repeat. Killing the zombies and reloading should take about three or so minutes all together.
If you're being efficient this should take well under thirty minutes to complete.