Rule 465.2.c.10
If a unit cannot be dealt damage, then no amount of damage can be considered lethal. Such a unit is exempt from any considerations of mandatory assignment. Example: Kayn, Unleashed says “If I have moved twice this turn, I don't take damage.” While Kayn can’t take damage, it is ignored for the purposes of assigning lethal damage in combat. Example: Counter Strike says “Choose a unit. The next time that unit would be dealt damage this turn, prevent it.” The unit in question has a replacement effect applied to it, but it still can be dealt damage. Thus, it is not exempt from considerations of mandatory assignment.