The halo 3 bubble shield is an extremely versatile piece of equipment that can provide you with instant cover and allows room for a lot of strategy. It can be a real lifesaver and the difference between a win and a loss.
The most important thing to remember is that the bubble shield will deflect all enemy fire, grenades and projectiles. However, enemies can walk right into it, vehicles can storm right through it and enemy equipment can pass into it as well.
If you have a energy sword/gravity hammer, then you are golden. Have your secondary gun equipped and deploy the bubble shield, as your enemy is charging into your shield quickly switch to your hammer/sword and lunge at them. The reason why I recommend the energy sword and the gravity hammer is because they have a lunge and you can be on the outer edge of the shield still protected and lunge out at them if they try to back off once they see your newly switched weapon.
One tactic I like to use with the bubble shield can be an amazingly effective fight stopper if you are losing - On maps like construct with narrow hallways you can sandwich the bubble shield in between the two walls, deploy the bubble shield, step outside of it and throw a frag grenade at the foot of the shield and watch your enemy charge right through eager to get the kill. If the explosion doesn't kill him, your next BR headshot will.
If you have the power drain it is AMAZING for countering bubble shields. If your opponent is just camping inside a bubble shield with a power weapon, deploy the power drain in there. He will immediately lose his shields and from here if he stays inside he will not recover his shields, the explosion from the drain will take down the bubble shield and you can finish him off with a clean battle rifle shot. If he does run out of the shield once again you can finish him off with a clean battle rifle shot.
The bubble shield is also very useful against that annoying guy that always rushes rockets every game. This is really only effective at short range but the second he shoots at you deploy the shield and the rocket will bounce back in the direction it was fired from. This is a great way to not only force a suicide but also acquire rockets for your team and score a few quick kills.
The bubble shield is extremely effective in oddball games. Simply deploy one and camp inside for a guaranteed 15 second gain. If anyone comes in, you know what to do. When you sense the shield is about to run out, quickly run and crouch around a corner. The shield will still protect you from shots for the next second or two allowing you to get a clean getaway and then you can smack your opponents once again as they come running in looking for you.
The bubble shield can come in handy on neutral assault - has the enemy armed the bomb? Deploying the shield around it and camping inside with a power weapon all but guarantees a disarm.
One other neat trick is if you have the sniper you can align a headshot on a person camping in the bubble shield with the sword/shotgun and pull the trigger the second it comes down.
There are lots more uses for the bubble shield than I can possibly list here - like I said it is an extremely versatile piece of equipment. Try some of these out and work the bubble shield into your game play and you're one step closer to becoming a pro.