Unlocking Overwhelming Force

[Juge]

Hello, I quite don't understand the wording of the mission and what I have to do in order to trigger it. Can someone explain with simple words :D
"Start the combat with 9 groups" : what are those groups ? Is it military cards (but we have only 8 in hands max), is it triggered by Rage (I understood that Rage is creating additional units, but I don't understand how they are stacked, or create groups).