They cross the border illegally, do the job and cross back.

If its not a VIP with protective service, assassinate a normal person is quite straight forward affair.

Someone tried to kill one of my sub-contractor many years back, he was extremely lucky to escape out of pure luck.
The killers on bike just beside him when his car stopped at traffic light, one bullet went into his thigh, another deflected off the steering wheel and the third one lodged into his shoulder as he turned away covering his head. Police told him killer gun likely jammed after 3 shots since he was still alive.