This foe, like most flying enemies, moves around in the air, at a slow but steady pace.
When it spots the player, or a creature it is aggressive towards, it will first attempt to make it target, using a special projectile. When the projectile hits, the marked creature will be a target for the next 10 seconds. During this period, all creatures in the world (with the exception of minions), regardless of previous relations, will attempt to attack the target. If its target has already been targeted, it fires fast blue bolts at them.
This means that you can trick the Observer into making a previously allied creature a target. Additionally when targeted, enemies will be able to see you through invisibility.