This page of the FF7 Remake guide has a description of The Angel of the Slums side mission, available in Chapter 8 - Budding Bodyguard.
First, go to the community center and talk to the residents. No one will want to give you information about the Guardian Angel. Talk to Mireille outside - go back to the journalist and you will learn important details about the mysterious character.
Go to the designated place, and you will face Chromogger. The enemy attacks with a giant mace, casts lightning and can cast Silence. The best tactic is to attack, jump backward and order Aerith to heal Cloud. Lightning attacks, especially Thundara, are also a good idea.
After the fight, go to the journalist and complete the task.
Reward: 2.000 Gil.