Most people have fiqured out that in Onslaught mode, pressing the Rocket Launcher? key (the "SwitchWeapon 8" command, bound to the 8 key by default) will select the Sticky Grenade Launcher or the Flak Cannon?, depending on which weapon you have. If you have both the Flak Cannon? and the Grenade Launcher, every time you press the key it will switch between the two weapons.
However, if you don't mind manually editing your user.ini file, you can setup keys to directly select the Grenade Launcher. The following command will select the Grenade Launcher:
GetWeapon Onslaught.ONSAVRiL
You can assign these directly to keys as in:
E=GetWeapon Onslaught.ONSAVRiL
or you can assign it to an alias and assign the alias to a key, as in:
Aliases[32]=(Command="GetWeapon Onslaught.ONSAVRiL",Alias="GetAVRiL") ..Further down... E=GetAVRiL
The key to using the AVRiL, is to not lock on to your target right away. I like to fire a rocket in the air above my target (or possibly below if you are firing at Raptor), then fire another if I am far enough away. When the rocket gets close, then I lock on to the target so the guy has only a few seconds warning :)
Its worth noting that this tactic can be countered by exiting the vehicle before the missile gets locked to it - you can't initiate a lock on an unoccupied vehicle. Oh, and another note for the n00bs. Use the secondary fire when you have a lock to prevent the lock from being broken when you move your aim away from the vehicle. Otherwise, you'll need to keep a perfect bead on your target.