How to increase the immunity of troops?

Immunity comes from the Virus Research Facility Doomsday Honor Structure. The green building that you build in the world map for the doomsday event. Upgrading this will give you more immunity.

You can also build more of them if you use your honor speciality points that you earn during the event in the Blue, construction speciality, section of the Speciality tab of the doomsday menu. Unlock your way diagonally towards the items called Construction Master, there are 4 of them but it takes 6 points to reach 1. Once you activate these you can build an extra Virus research facility (and other buildings) for each cone you get to. The 3rd and further buildings though need to be built next to other alliance centers. (AC2, AC3 etc)