As with all the Halo games so far, regardless of editions, there are ways of bypassing pieces of the level. For Halo 1, there are quite a few; the most easily accomplished I've gone and listed for you peeps.
Bypass one: Once you get the Warthog, turn around the way you came in it and go back that beach; you will not encounter any enemies until you have to go in towards the middle of the island. (25% of level bypassed roughly)
Bypass two: After deactivating the security system, turn and engage all enemies in the room (namely the two active camo'd Elites), allow them to drain the overshield (if you have one) then after defeating them, go outside, retrieve an overshield power up and proceed to slide down the cliff. How? If you step to edge and look down, the first outcropping is visible, jump to that, then edge to the left a bit, you should slide down. (25% roughly skipped)
Attack on the Control Room
Cheat/bypass: Depending on your skill level, whenever you encounter the 2nd downed Pelican, grab a rocket launcher and the ammo for it. After taking care of the two Ghosts that come out of the tunnel and to your left (coming in from the 1st Pelican), walk over to the left ridge (two Jackals and one rookie Elite will spawn there on easy), kill the enemies, then pull out the rocket launcher; aim up. Fire up at the ledge you'll see through the scope, if you do it right, a Banshee will fall off said ledge from the firing. You'll be able to keep the Banshee up until you have to get out to go through the door leading into the next indoor part from engaging the Covenant on the ridge. (Cuts 20-35 minutes off level)
Cheat/bypass 2: Once you come to the two active camo'd Elites on the last indoor section before you exit and are outside the control room, kill both Elites and turn to your left. Third jutting down (on the wall), there's an active camo powerup. Take it, and run through the room with the Hunter pair. Run through the corridor, jumping over the enemies, and if done right, you should only have to contend with one Elite head on. If successfully done, you'll get a Banshee, because the Rookie behind you (seen coming through the door out onto the rock bridge) will not take the Banshee.