I don’t think difficulty inherently scales with co-op, but for whatever reason, only in Halo 2, if one person dies you restart the checkpoint. In the other Halo campaigns, as long as at least one person from the group is alive you can keep going. Respawning only happens when there’s someone out of combat but you don’t restart the checkpoint unless everyone dies before someone can respawn. Halo 2 has no respawn, which actually makes it harder in some ways. Especially with those snipers. But obviously having a second (or third or fourth) gun to shoot at snipers does also make it easier. It’s a trade off I guess.