Halo Campaign Evolved: This Is My Ship Now, the Easy Way
This Is My Ship Now sounds like a fight. It's actually closer to herding cats, if the cats couldn't die and you were allowed to punch them the whole time.
The setup: kill Captain Keyes, trigger six invincible Marines who want blood, and shove all six into a specific room before they catch you. Written out like that it sounds like a nightmare. In practice it takes under a minute once you know where to push them.
Where and When
This one's tied to The Pillar of Autumn, the very first mission, and it unlocks at the Rally Point Bravo checkpoint. Run it on Easy. The Marines can't be killed no matter what difficulty you're on, so the only thing difficulty actually changes here is how much damage they land on you while you're setting up the kill. Lower difficulty just means fewer headaches during the part where you're not in full control of the fight yet.
Killing Keyes
Walk up to Captain Keyes and a cutscene plays out. Once it ends, leave the room entirely until your Magnum is available again, since you can't draw a weapon on him mid-scene. Head back in and unload on Keyes until he drops. The moment he dies, Cortana locks the ship down, and six Marines spawn in ready to avenge him.
Trapping the Marines
Here's the actual objective: get all six Marines into the observation pen, the small room at the front of the ship where the two pilots sit. Don't try to stack them by the ladder. That's the wrong spot, and it turns a simple task into a longer one.
The trick is melee. Punch a Marine and he doesn't just take the hit, he skates backward across the floor from the force of it. Keep walking into him while you spam melee and he'll slide the rest of the way into the pen almost on his own. Repeat that six times, herding one Marine at a time or bunching a few together if they're standing close, and you're done. No precision required, no timing window to miss, just persistent punching in the right general direction.
Once the full group is inside the pen, on either side of the pilots, the achievement fires immediately.
If this is one of several achievements you're chasing down in one sitting, our complete achievement roadmap lays out the most efficient order to knock them all out.