I'm presuming you're asking for help on how to kill the bloody Mothrakk!
The easiest way is with one of Scooter's runners. If you choose the rocket launcher, you can go relatively close, switch to the gunner seat and take pot shots at him - dodgy AI means that he will not attack you even when you hit him. However, this takes a while as you have to lead the Mothrakk, and the rocket's don't do a lot of damage. Allow about 20 minutes to kill him this way.
If you choose the runner with the machine gun, you can lock on from quite a distance away, but it is better to aim manually and lead him from the gunner seat. Whilst this is much more effective and quickly takes down his health, he will attack you this time with a hail of fireballs. To avoid this, when he begins his run-up towards you, quickly change to the driver seat, hit the nitro and swerve around. After he drops his salvo, drive back to where you were and continue firing. Allow about 8 minutes to kill him this way.
Note: For both these vehicle methods, demolish Mothrakk's health with the rocket launcher/machine gun, but try to actually kill him while out of the vehicle. If you've got a half-decent sniper rifle, just unload a clip into him when he's almost dead, and you'll get much more experience than if you killed him from within the vehicle.
You can also try to take shelter in Bruce McClane's hut and shoot at him while he's not attacking. Personally, I found that this was too annoying, but I've heard that it does work.
Hope this helps.
If you use the above method you can shoot at Mothrakk from inside the driver's seat. You do not have to switch seats. To lock on to a target from the driver's seat simply hit the spacebar. This will automatically aim your gun at the enemy. To fire simply hit the right mouse button.
Also, any Rakk in game can be easily killed with a Shotgun. Mothrakk is the same. Just get a decent shotgun and take cover at scooter's hut and you'll be able to take him down quickly enough.