After experimenting with a couple of different strategies, I found this to be the most effective Halo 2 strategy. Mind you, it's mostly for offense in CTF.

To begin the game, have each teammate get the weapon for their role(i.e. sniper). The sniper should first try to pick off at least two guys within 15 seconds of each other (hopefully one is the enemy sniper). Once he does that, send in one battle rifle guy and the close-range guy to their base.

The second battle rifle guy should move into a position where he can cover the sniper and still lay down some fire at the enemy.

Oh yea, if you're playing 2 flag CTF, have the sniper cover the flag. Make sure he has a close-range weapon though. But hopefully, the enemy will never get that close to him.

Next, have the close-range guy go into the base or wherever the flag is. Don't crouch, just run towards the flag. The first battle rifle man should be following him, crouching. He should find a corner or something where he has a shot at the flag area, but not be easily seen, and should be positioned there before someone touches the flag.

Your sniper and second rifle man should move in a bit. If the enemy's sniper rifle is open for grabs, have your second battle rifle man get it. If that's the case, have both snipers cover all the exits.

Then, have the close-range guy go get the flag. His job is to just get the flag out of the base or into the open. The first rifle guy, who was in the corner, should start attacking the enemy at this time. Hopefully, he'll be able to get a kill or two since the enemy is concerned with killing the flag guy.

If the close-range guy had succeeded in getting the flag in the open, the first rifle man should get it and go. The 2(best-case scenario) snipers should cover him.

Unless, you're snipers are trash, you should be able to win with ease from there.

Even if you don't have both snipers, you should be able to win, it's just going to be a little bit harder.

If you wanna check out the defensive strategy that goes hand-in-hand with this strategy, go to Halo Insider.