Kayn Passive Problem

Kayn's passive reads that when damages after melee champs he gets Darkin points (because I have no other name for them) and when he damages vulnerable ranged champions he gets shadow assassin points (again see above) but what happens when you attack someone like say Gnar/Kayle who can be either ranged or melee but in neither case are they really vulnerable - see also Urgot who while ranged can be tanky enough to be a bonefied tank. Or assassins like say akali which are melee but somewhat vulnerable... what about champions which are technically melee but are almost completely ranged like or ...

Can we have some clarity of which is which?

Also while I'm posting there is a bug in the training where if you start a game transform reset game and hit a champion bot ONCE you will get enough points to transform immediately...