Thanks, guys. Halo 2 still seems harder than I remember it being, but you may be right. Other than Halo 4, it's got some of the worst checkpointing in the series.
You're fighting the flood. You literally cannot kill everything; they just keep spawning. If other levels were gladitorial arenas, The Library is like river rafting.
It's great because it requires players to use everything in their understanding of the game's mechanics to succeed, even thrive. It's a test of the player's understanding of Halo's mechanics. Most people who suck at it are players who haven't fully mastered the game's combination of movement, shooting, grenade use, and environmental control.