I managed to fight the wave on insanity difficulty, it was liturally insane, but i got a sense when i did it a few times, that killing all those around, banshees, brutes and other stragglers that the objective would be complete to fire the missiles,
so a run through
you need to kill everyone that is there first,
then another wave from the front comes, kill them,
then a wave from your left comes, kill them,
I believe it says your clear and you shoot the missiles, that is 2 missiles out of 6 gone
it then says that they are going to use the ship to increase the stability of the missiles, which results in having to kill another wave, a few brutes and banshees with this wave, after that wave, you shoot missile 3 and 4,
you need to get the reaper away from the beam, they say, this is the final wave, that seems to keep coming.
first its normal troops, its about keeping the troop numbers down, while you can see the reaper coming closer.
The closer it gets the harder the troops get, so about 5-10 mins before you get to shoot the missile, you get lots of brutes and banshees, about 5 banshees and 10ish brutes, kill them as best as you can, but try to stay close to the missile launch, then you get the all clear to shoot missile 5 and 6.
it then finishes, and goes towards the final, final assault lol
so overall its not just an endless amount of troops, its set in waves, like multiplayer.
Hope this helps at all, many thanks for reading