In multi-player mode, drive any Warthog close to a building so that the side of it is touching. Get in the gunners turret and turn it towards the wall. If done correctly and you are close enough, you should be able to to shoot at an enemy through the wall and not receive damage from them if they shoot back.
Good Sniping Position in MP
NOTE:-This works best in CTF when on the red team.-
On the map "Bloody Gulch" there are small dirt paths going up against the hill sometimes indicating to a cave. Now, jump into the Rocket Warthog and get a passenger (with a sniper rifle) and a gunner. Then, look for the entrance to a cave (hidden behind big rock) and drive in. Go to the place where the Flame-Thrower is and tell your gunner to take the over-shield (he'll need it!).
Now comes the tricky part: keep driving along the ledge towards the Blue Base and drive through the next cave. Stop your Warthog on the ledge where you overlook the Blue base and order your gunner to "snipe" as well as your passenger. The only disadvantage is that the enemy might have a sniper hidden somewhere (this is what the over-shield is for!). Now, you can pound the base with rockets.
This is also a good defense point; when the an enemy is trying to enter your base and steal the flag, BOOM! pound him/her with rockets!
Using Grenades (Hint)
Plasma Grenades for:
Grunts; when it sticks to them, they will usually run into their comrades asking to take it out (which is useful for killing great masses!)
Elite; will usually dodge them but if you get close enough you should try to stick it on them (should kill em' first time, if not they get pissed off!)
Hunters; they are slow at dodging em', but its useless (only if you want an angry Hunter charging you)
Flood; don't try it, if it sticks to them they might jump at you and bring you to your death. It is effective against small ones, but only if it doesn't stick to them.
Jackals; they usually ignore grenades.
Now, Frag grenades for:
Floods; These won't stick to them which is good against the small ones.
Grunts; Useful when cornered, but not very useful when in open fields (maybe against large packs otherwise they can easily dodge them)
Elite; Again, one should do the trick when cornered but in open fields its no good (they dodge them easily)
Hunters; Useful anywhere (they are slow at dodging them) but it will only get them angry instead of killing them.
Jackals; will usually ignore grenades, but be careful with frag grenades: they might bounce off their shield and come back at you.
Another tip when you need to kill great masses of aliens or in multiplayer games:
When throwing a plasma grenade on the floor, their is a 3secs pause before it blows up. Now, when wanting to spread grenades, throw a plasma grenade anywhere on the floor (preferably near your opponents) then quickly throw another right next to it. When the first grenade will explode, it'll catapult far away any other grenade near the explosion.
Hope this will help you. Have Fun!
Tips on using weapons
When fighting Covenant or Floods, try to spot which weapon is the most available. In some missions, for example, it might be the Plasma Rifle and the human Pistol. Once you've spotted these, make use of them. If you arrive on a mission and find a weapon but soon run out of ammo (or battery life) and can't find any, it could put you in danger. Especially in the heat of a battle. So keep in mind which weapon comes up the most in which mission so you can make great use of them.
Another tip, when battling (especially the Floods on 343 Guilty Spark) don't make use of all your grenades. If you see you can't get a clear toss, then don't waste one. Grenades are most rare when needed.
Assault on the Control Room Walkthrough
NOTE: Reading this walkthru will not get you through Legendary, it is done for Easy mode only. Also, I'm assuming at this part, that you have a Rocket Laucher (with 10 rockets) as one of your weapons.
Now to the walkthrough.
After defeating (or fleeing) the two Hunters in the room, you will get through a corridor and soon reach a door. At this point the "If I had a super weapon" part begins. Now, DON'T go through the door, now I'll tell you how to get a Banshee earlier than usual.
Pull out your Rocket Launcher and get ready to use the zoom. Then, strafe to the left of the door (facing the door and along the wall to the left). Next, approach the door until it opens. The moment the door has opened, quickly zoom in and shoot the Elite in the background (the one nearest the Banshee). If done correctly the Elite should die and the Banshee will stay there.
Make quick work of the second Elite as he could steal back the Banshee. Then, make your way towards the Banshee while shooting the scrambling Grunts. Defend the Banshee until there are no more enemies nearby.
While you were busy shooting up the group of Grunts, another Elite took off in a Banshee and is now shooting you. Pull out your Rocket Launcher and shoot down the Banshee (don't take off and start a dogfight, you might lose too much vehicle health).
-If you couldn't get the Banshee, scroll to the bottom of this text. Otherwise, carry on reading.-
Now that the enemy Banshee is out of the way, take off in the captured Banshee and make your way to the top of the Pyramid. Land close to the door control and open the doors. Now, if the Banshee is still pretty fit, take off and shoot a Fuel rocket (right mouse button) at the yellow Elite with the large, blue sword before he jumps on top of the vehicle. Make quick work of the Grunts and fly in the long corridor. At the end of it, two Elites are waiting. Get rid of them and land your Banshee.
Activate the last door opening and fly to the very last door (the one that triggers the scenery) and land. Open the door and watch the movie, you then move on to "343 Guilty Spark".
-If you didn't get the Banshee, read this.-
If you didn't get the Banshee, proceed all the way across the bridge and kill any enemies. Make you way into the structure and kill (or evade) any enemy inside. Find your way to the elevator which will take you down to the ground floor. Again, eliminate any threat on this floor (or evade them again) and make your way outside.
You should see a patrolling Elite and a Ghost. Cortana will hint you to commandeer the Ghost as you'll need the firepower. Try to sneak up on the Elite and kill it. Get the Ghost and make your way up the Pyramid. Kill any enemies along your climb and replenish yourself with ammo along the way.
Once you get to the top, you should be facing 3 Shades (empty, phew!). Soon, a Grunt comes down and attacks you. He is soon followed by another and, UH-OH! Two Hunters rush down towards you. Pull out your Rocket Launcher (right now I'm praying you still have ammo for it) and get rid of them ASAP.
Then, make your way up the slope to the door's control panel and open the door. Now, don't wait there and think you've passed the level. Run and duck behind one of the energy shields facing the door. Once your behind it, you will be 'invisible' to any Covenants. Take the time to spot and aim at the yellow Elite with the huge sword. Then, you can either toss a grenade so it sticks to it (and kills it) or shoot it with whatever you've got (basically, get rid of him FIRST).
Once you get rid of the Elite, take out the Grunts, replenish your ammo/battery life or grenades and make your way to the end of that corridor. You should bump against two Elites, get rid of them and then open the door. Make your way through the last stretch (no enemies here, as Cortana says) up to the very last door (the one that triggers the scenery).
Congratulations! You've finished the level, the scenery starts and then you embark on the next mission: 343 Guilty Spark.
Hope this helps.
Another way to snipe in Sidewinder
Here's the trick: Drive a Warthog up to one of these caves with the teleporter inside and park the Warthog with the back (gunner platform) facing the wall which is facing the outside boundaries. Get out and jump onto the platform (don't climb aboard, simply jump on it). Next, facing the wall nearest the back and toss a frag grenade so that it bounces off the wall and under the 'hog. You should be catapulted on top of the map and will be able to walk outside the boundaries.