Banshees really need a transport in order to make them effective, otherwise they will never get into close combat, they will be a splattered smear on the table due to gunfire.
If you are playing Marines regularly get some Harlequins and tool the majority of the squad up with kisses to get rending attacks. They are extremely mobile and suprisingly tough to kill if used right. They will take practice to get good with though. I\'d suggest these dudes instead of Banshees any day to be honest.
Shining Spears are also very good against Marines as long as you get the charge in, but with their maneuverability that shouldn\'t be tough.
Basically you want to get as much in your force that is AP 3 or better, or ignores saves.
The Wraithlord and Avatar combo is a good one and should be used along with the Harlequins so that they gain fearlessness from the Avatar.
You could also stick in Eldrad to really make this group really devastating. You can use him to reposition troops at the start of the battle, so can move this elite block of killers to where they need to be. As they advance cast Fortune on the Avatar and Harlequins to keep em safe (veil and rerolled 5+ invulnerable save). Eldrad should hide like a coward behind the Avatar and Wraithlord to keep him safe, and once your units are in charge range he should start to storm units and vehicles to pin or spin them. It is a bit of a cheap tactic if you keep doing it, but effective (the team who won the 40k doubles tournament this year basically used this as their strategy).