Every time you defeat one of Elizabeth's lieutenants she will send a retaliation strike after you. There will be a cutscene before each of them. Just kill the waves of enemies she sends after you.
Edit: It's been brought to my attention that this is missable if you kill Elizabeth. If you do, start a new world with the same agent, it shouldn't take you more than 15 minutes or so to defeat a lieutenant while using an overpowered agent.