In order to get this achievement, you'll need to heal the technician when he's injured. If there are any zombies around, he'll be considered "in combat", and you won't be able to heal him.
In order to heal him, you'll need bandages, which you can find in the storage containers in the back, and also from Cleo drops between waves. Run up to him, hold
, and as long as you have a bandage in your inventory, and as long as there are no zombies around, you'll heal him. Do this 12 times (you can only heal him when he's not at full health), and you'll get it. For me it didn't seem to track when I was in MP with another host, but it tracked when I hosted or played alone, so that may be a requirement.