Massed enemy in the boarding room
Regarding the boarding battle on the ship, fellow cyborg Doug Auclair brought a novel plan to my attention, which you may have some fun exploring. Leaving your Marines to be rapidly wiped out, run into the passageway joining the two interlinking exits as soon as you can, and hide there. In a situation reminiscent of how the rockslide megabattle is formed, the enemy continues to arrive in batches until you've got the whole lot milling around. What you do with them is up to you.
The obvious plan is to just wade in and see if you can "stick it" (as Cortana might say). You can lure the Elites up into the corridor to kill them first if you like. But here's another challenge. Get the active camouflage and then try to melee attack everyone to death before it wears off. However, on Heroic or higher, you'd better kill a few of the enemy near the camouflage before making your dash for it, else you tend to be under too much fire too soon.