I'd suggest any time after you've completed Operation: Heaven Sent, open your map and scan the surroundings for any orange targets that aren't: LEGION Outposts, Relic's Stands, or Gremlin's Hideouts. If you've taken back the outposts recently, you might see one icon that is different from the others; this would be a Golem, similar to the one you fought early in the first chapter with the Franchise Force agents.
If you use someone, say Braddock, with a vulnerability Special, you can knock down the 32k health that it has (or at least that's how much it had for me). The reason I suggest it being a bit later is so you can have some of the later agents join in and use their abilities to the full extent.