Banshee defence

Use the Banshee to support your Marines or fight solo. You could hover over the battle area and bombard the enemy, or make more dramatic swooping attacks, perhaps including a bit of squashing.

Squad size consideration

If you've got a load of Marines, the fighting down below can make quite a spectacle with all the plasma and whatnot flying everywhere, and it's interesting to see chain reactions from above. However, if you have high enemy numbers and also a large squad, framerate slowdown can make your Banshee feel rather less fluid than usual. If you want the smoothest flying I'd recommend having only a small squad or none at all.

Difficulty levels

Concerning difficulty levels, Heroic is very unforgiving if you get too close to the enemy. With high enemy numbers the fire can very quickly turn your machine into a smoking wreck, and there may not even be enough time to react and get clear if you misjudge an attack run. You have to be very careful to minimize incoming fire. If you like dramatic low attack runs, it would be best to set things up on Normal or Easy to give yourself more chance to enjoy things. That would also help Marines survive longer. Another way to help them would be to have someone on a chain-gun. You'd need to think about where to position him in advance.