Took me several hours to complete. So many times I found myself at checkpoints with very low health, meaning I died loads of times.
There is a trick you can perform at the first bridge to get a hold of a banshee and despawn all enemies for the rest of the mission. If you search CE banshee grab should be able to find a tutorial video.
There are many different strategies that you can use in different places. As you play on legendary more you learn how to always land rockets on Banshee’s, how to noob combo, how to choose weapons and conserve ammo. It just kinda takes getting to know the game.
One thing that I do to make the level significantly easier is to grab and keep the Sniper once you get outside and on the ground level (near the flipped Warthog with the marines), you get plenty of Sniper ammo for the remainder of the level, definitely helps out with the double bridges and the final part of the level.