In order to execute the "Bonds of Battle," you must first have all of your party members. Clarus is your final member and won't join you until you enter the cave (the stage after Duscae). Once you've got all three with you, build up your combos to execute each of their combo attacks. You'll see a medal appear over their portrait once you use them, signaling they are ready. Once all three are set up, execute Armiger with
and you will have performed this.