Ceiling Flood attack
Shortly after re-boarding the ship, you drop down through a hole. There's a right turn with Flood ahead, then a left turn with covies ahead (they clash with the Flood), then a right turn with covies at the bottom of a sloping passage. This final group isn't very alert, and it's usually easy to take them out with a sneaky grenade or two if you like, perhaps tagging the Elites.
But now here's the bit I want to talk about. Just as you reach the bottom of that sloping passage, you trigger new Flood (and potentially a checkpoint if you pause long enough before heading into action). A few spawn behind you around a corner and tend to just hang around. But just ahead, a mass of Flood is about to start dropping down through an opening in the ceiling. You could easily just run past the opening and speed on to the hangar, and maybe that's what you normally do. But why not stay around for some fun? Armed with a shotgun or whatever, run past the opening as far as the corner, turn around, and see if you can defend from there without getting forced back through the nearby door. You can blast the Flood as they land, or even before they're down. The occasional grenade can buy you a bit of reloading time when needed. They spawn thick and fast, making this a great little close-range blast. It can be quite a challenge, depending on your weapon combo and the difficulty level you're using.
You'll probably have got a checkpoint just beforehand, which allows you to repeat the fun as desired - perhaps varying your weapon combo. In fact it's quite possible to set things up so you've got a collection of weapons to choose from just as you get a battle start checkpoint; but I'll leave that to you.
Of course, there are other places in the level where you can experience repeated Flood spawning to attack you. Sure you may be able to run past; but just as with this ceiling attack, you can potentially have a lot more fun by hanging around to try and hold out against the mob.