The crimson prototype is pretty big and can move around so maybe the difficulty targeting large enemies has been solved.
As for reworking the fight, i wouldnt go with require killing the nictus first as this is one of the few bosses with a choice for how to attack. Maybe change it up a bit depending on which route you choose, but dont stick a roadblock on one of them.
Maybe the robots could drop some orbital bombardment from somewhere outside the map (but werent we disabling them in the previous mission?).
I would like to see more 'do something in a fight or bad things happen' but i know alot of people are against that. Move out of the death zone, close the enemy spawners, jump in water to remove some DoT, man the turret to fire some missiles. Things to distinguish boss fights from other boss fights. I think the Romulus fight is already distinguishable though. Id like to see some of the other bosses get a revamp first.