bring an aggressive NM to the person you want to kill.
ex. Silk Caterpillar in Rolanberry spawns every time an airship flys overhead. It's agressive and can be brought to the zoneline where it will kill bazaaring mules on it's trip back to the spawn area.
Also can be done with Sea monks on the ferries, but require either Charm/release or using a warp item.
Note: a successful mpk will likely result in a ban or temp suspension.