The Upper Cathedral Ward is a dangerous place, but before you can rest easy in the Lumenflower Gardens, you'll have to defeat its guardian: the Celestial Emissary.
This fight is slightly different from others, in that it's more of a mob boss. When the battle begins, several of the big-headed blue aliens will spawn in. They can be fought like normal enemies, but one of them is actually the boss - you'll know when his big HP bar starts going down. Whale on him like any of the other aliens, and he'll only fight back with the puny punches the standard minion throws. Continue to take out the aliens as they spawn, focusing on the ringleader.
When he's at half health, he'll grow several times his regular size. The fight plays out the same, but he'll hit harder, take less damage, and have more reach. Keep this in mind as you continue to damage him, being sure to sidestep his punches and arm slams.