Once the mission has started , head to Test Site Jefferson. Approach the mission marker on top of the building to update the mission. Next, head to the site and take out the blackwatch defenders.
After clearing the area, enter the building next. Consume the senior scientist then clear the room. Open the next room to find the Juggernaut. It deflects attacks and the best way to defeat it is to grab a rocket launcher, wait for it to pick up a debris that it will throw at you or when it raises its arms to smash, this shoot it while its vulnerable. This enemy requires timing to defeat as its not that hardy compared to the brawler.
After defeating it, you'll obtain its shields. To complete the bonus objective, raise your shield before the enemy rockets hit you to deflect it back to them. Once done, exit the test site and escape the alert to complete the mission.