What constitues a "Checkpoint" in Halo ?

I'm playing on "legendary" level, and I'm at the bit just before the Gravity lift to board the Truth and Reconciliation. For some reason, the game saved a checkpoint right in the thick of some major action when my health was as low as it could get, and I just can't get out of this area. If I go forward I get blasted by sentry guns, if I go back I get shot by 4 elites and a load of grunts. If I stay still and fire and throw grenages, I last for about 10 seconds at the most. I thot that checkpoints were saved when certain tasks had been completed, like killing all the enemies in a location. I accept that "Legendary" should be challenging, but i never asked it to "save" at that point, and even the most challenging games should have an element of fairness. Is there a way out, or do I have to re-start the level ?