This is the main story to the game. Each operation is broken up like an episode, so after a few missions you'll get the achievement. These are:
-Beam With Pride (Boss Fight Part 1)
-Skyfall
-All You Can Eat (Boss Fight Part 2)
The boss fights are pretty straight forward. If you find you're having an issue, just lower the difficulty and come back to it later when you've seen the whole fight.
After those 3 "Legion Operations" you'll have your self a shiny new achievement.
EDIT: Its been a while since I played, sorry I don't have a more detailed understanding of each fight. I don't remember this game being the most challenging. Frustrating at times, but not challenging.