Unless there is a large amount of infantry who can inflict large damage to you, let your gunner eliminate them. Vehicles, if present, are always your first priority.
You should always try to give your allies good weapons, such as the M41 rocket launcher. A good configuration, especially in tight quarters, such as those offered by Kizingo Boulevard, is to give the rear-most Marines Rocket Launchers, so they can destroy vehicles, and the other two Marines either the MA5 assault rifle or the M90 shotgun, if possible, so they can kill any enemies which get close to the hull.
On higher difficulty levels, the Scorpion is highly vulnerable to being EMP'd by a Plasma Pistol. Any sign of anything charging a charged Plasma Pistol shot should be eliminated as soon as possible.
Another popular tactic is to use a "hull-down" position. The tactic essentially requires the tank to be positioned such that its hull (the main part of the vehicle) is behind a crest or other raised ground, but its turret (or a superstructure or roof-mounted weapon) is exposed.
Equipped with a powerful gun, the Scorpion can easily take out any aircraft it lays its eyes on. A way to counter this is to position your aircraft directly above the tank where its "blind spot" is. Then lower the aircraft on top of the tank keeping sure that you're still in its blind spot. Then, get out of the aircraft and jump on it and stick it with a grenade effectively destroying the tank. This tactic is primarily for use by the Hornet and Falcon with them being the only two vehicles with VTOL technology. This tactic can still be employed with the Banshee, but with a less effective outcome.
Using a Power Drain or a Type-25 plasma pistol overcharge will temporarily disable the tank's mobility and will greatly weaken its driver. While the tank's weaponry will still be functional, unless it is Halo 4 multiplayer, it will be unable to move and can be easily boarded.
An effective hijacking technique is to use Active Camouflage and wait for the tank to pass-by or approach the player.
On Sandtrap, a good tactic to hijack a Scorpion is to wait for the tank to go into the middle depression and board it from above.
In Halo 4 it is very difficult for a Scorpion to hit a Type-26 Banshee due to the 90mm cannon's slow velocity and arc. A Scorpion should always lead their shots if they have any hope of taking down a Banshee.