This will be your final weapon, which you will automatically equire during the last mission from your new BT.
As you have the Legion auto equipped prime your smart core to 100% by killing or shooting at other titans. There will be multiple enemy titans to hit, which appear almost straight after jumping in your new BT.
Once your core is at 100% ensure you choose a titan which has low health or lower it ready, then push
on the D-pad whilst aiming at another Titan to kill!!
Pop achievement unlocked and congratulations!!!!!