The problem is, the payoff for that tactic (assassinate + cutoff retrest) is very low. Against weak armies it works, but if the target is juicy at all, it is probably led by some beefy commanders, who would laugh in the face of assassination attempts (Especially with bodygaurds).
It has low risk though, as he would have attacked you to get in that position anyways, so you know his commanders and you can probably see if he has troops with "gaurd commander" as well.
Just watch out for any of HIS flying commanders to swoop in and a big boost of PD. The plan could backfire into forcing the enemies hand to create a big fortified province in the center of your lands
.