Slayer Task kills count in 1 of 2 ways. Either you deal the most damage, and get half the xp, or you deal the finishing blow, and get half the xp. Do both, and get full xp. (This is why some players are willing to leech off your cannon--they can run around dealing finishing blows and get xp pretty quickly.) So by that logic, the easiest way to do this task would have a friend pwn the KQ, then you run in and deal the finishing blow. That's assuming, however, that the kill counter works the same for special tasks. If not, then have a friend deal just under half the damage, and you take the rest. I believe special tasks require you to do majority damage. I'm not sure if the xp is handled the same way, but I wouldn't be allowed to do only the finishing blow.