Into the Belly of the Beast is a tough fight, but it's easier if you play it really aggressively. That's true for a lot of fights in Halo 1.
Lay into the enemy as they're coming through the doors, using the motion tracker for information about where they'll come from next. Try to rip apart groups of grunts and jackals as fast as possible; AR and PP can work here, and grenades if you desperately need to blast a wave.
If there's a group coming toward a door and one blip is way in front of the other blips, there's a fair chance that a sword goldie is coming. They like to roar, which is a great time to stick them with a plasma.
When music plays, you're down to the last few waves.
Try to keep killing enemies quickly to avoid being overwhelmed as long as possible. When you absolutely cannot sustain that any more, the next time you're by a door with enemies, get to the door. Only enemies can open them from the front, but you can use the hallways+doors for cover and to funnel enemies, to slowly pick off the last of the enemies. It sucks to be in that position, but you'll typically have to do some of it unless you're doing
really good.
