The problem with kills is that it would be very unbalanced. PLD could equip Excalibur and Aegis, while MNK can only equip Spharai. And don't think you can come RDM/NIN (Excal+Mandau) either, because ADL doesn't strike me as a NM you want to PUG...
Oh, and don't forget you need to be alive for kills to count...
Magian trial could give an all job ranged slot item to record the kills for the particular trial you wanted to do, when completed you trade the weapon and the ranged item to the box to complete it.