As mentioned before the Green Baron is a single death copter that has twin linked rockit launchers and a buzz saw.
The green baron's main job is transport killing, get behing them and shoot it, and if the rockit fails to hit then assault it with a power klaw equivelent (the saw).
Two tactics are: First to turbo boost during the scout move phase to get the 3+ save and to get closer to the enemy. Then move into the enemy line and shoot/assault vehicles, mainly anything with a rear armor of 10, they are not effective against land raiders or monoliths. Second tactic is to hold it in reserve and when it comes in, to outflank the enemy and do the same thing; shoot/assault vehicles. This is especially effective against the IG and all their pie plates of doom vehicles.
In practice they either do really good or get shot down early. Regardless, the green baron will draw fire and rarely survives a game.