To complete a Lightbringer all you have to do is complete one tear and escort. Which is just walking the light back to spawn, once there you can hold B to activate the beacon and start the final engagement. Once you kill everything and deposit spawning crystals 3 times and kill the mid boss it will be complete. But I recommend pairing this with finding and opening 200 light chest. This can also be done solo, just make sure you are Lvl 50 and have a decent build.