I only remember two or three doors in Act 3 that Jack has to open. One in the rocket hangar where a bunch of Juvies, Poppers, Grenadiers and some Elite Hunters show up as well as a Warden, one in a side mission with loads of Hunter Elites and Juvies(NOT fun!), and one when you’re in the cosmodrome in the Windflare with a bunch of Drones, Poppers(both easily killed with the provided turrets) and two Boomshot Scions shielded by Bastions(by far the most difficult to deal with in that section due to the unpredictability of the shots they fire against the wind when you’re on one of the turrets to kill the Bastions, and how the AI frequently lands magic shots right on their first aim attempt, and because they will separate and kind of pincer you there if you don’t manage to take them out quickly).
Being honest, other than the encounter with the Warden, I found the door outside the mine in Act 2 to be far more difficult to handle, especially on my first Experienced playthrough(why did the difficulties require renaming again, rather than just adding the new names on top of what already existed?). Even if you managed to use the ice lake to your advantage(hint : look above on the train bridge for something big to drop into the lake, or use it later to get “Oh… oops” from Kait, but it won’t serve any use then - I discovered that by accident when some stray bullets hit something when I was aiming for Juvies after leaving the mine with the phosphorus), you could still end up with a Bastion shielded Grenadier flanking on the right while there is Poppers closing in, or barring that, the Bastions will attempt to kamikaze you if there’s no enemies left for them to shield. Took me so often on the first playthrough… but on Insane, I beat that part on like, my first or second attempt.
I still question why we get such a ridiculous Horde/Escape balance(more in terms of player health in the latter, the enemies don’t have as much health in Escape) when they have managed to make the Campaign a lot more bearable.
PS If you missed the Relic EMBAR, it can be found near the crashed Condor in the open world section of Vasgar by the turntable. At risk of spoiling its potential, I was able to one shot kill a Scion on a headshot with that thing on Insane. Crazy high damage, but long charge up.