When dealing with instances of the Strongest creature, how easy it is to confront that creature is not factored in. If the strongest creature is a flyer, then the only way you'll get the bonus is if you can manage to attack the flyer. If the strongest creature is walled off in a corner, then you'd have to find some way into that corner.
A realistic example would be a Priest that might have a bunch of level 1 Clerics plus a Crusader Griffin as a Holy Avenger. The Knight of the Red Helm would only get the bonus against the Griffin, despite having a very hard time actually hitting it due to the Flying.
I wouldn't say never, because you can get the bonus on Counterstrikes if you use him to guard, and you might be able to find a way to counteract the flying like with Gravikor, but in general you are correct that if you can't hit the Strongest creature, for whatever reason, then you can't get the bonus.
