I note, with some dismay, that a grappling hook exists as an item without any notes whatsoever. In the last room of Siege of Borodin's Watch, my scout decided to climb a thrown grappling hook to the top, rather than take the long way 90' up. How should I determine the difficulty of this throw?
The grappling hook is part of the climber's kit, which as a set of items collectively gives a +2 bonus to Athletics checks to climb. Effectively this seems to be intended to be abstracted out. You set the DC to climb the wall, and the climber's kit gives a bonus to the check.
However, if a DM chooses to, or if the players would rather abstract it this way, you can make each piece of the climb a check. Rather than use an attack, make it an Athletics check and set the DC; accuracy isn't all that important, it's more about distance and care. It's 18 squares straight up, though, so it's beyond the range of a typical throwing hammer or dagger (light or heavy thrown weapon, anyway), so the DC for the toss may be fairly high. However, the kit includes pitons as well, which could be used in the wall. This would be a series of Athletics/History/Dungeoneering/Nature checks that could make the entire climb into a very fun and interesting skill challenge.