This can only be achieved once you've met all three companions: Weskham, Cid, and Clarus. You will meet Clarus in the final area, meaning you must complete the majority of story mode before you can attempt this.
Once you've met all three companions, start building up your combo. The final levels of the game can become hectic, but if you time your dodges with
, you shouldn't have too much trouble keeping your combo alive. Once a companion attack is available, use whichever you choose (
), and continue building your combo until you have used all 3 companions.
By this point, asssuming you haven't been hit, you will be able to use the Armiger with
. Take note of the emblem that appears over each companions' image, and make sure that all three have this emblem. Once they all do, and you can use the Armiger, use it to activate the Bonds of Battle. Successfully activating the Armiger while all 3 companions have been used in the same combo string will unlock this achievement.