At 65 it will be almost impossible for you to 5 man the last boss...

There are numerous posts/articles about this last quest and even with an extra 70 helping out on top of the 5 65s it will be tough. Level up a bit or see if you can get another multiboxer to help or maybe 2 70s (maybe just 1 70 if hes a well geared tank - anything other than a tank and you'll probably need 2).