It's not that thick. The fallen log is only a couple of feet thick at most. You could hop up onto a tree limb and attack the bat as it passes by (and also potentially get a couple of AoOs on it), but you'd need to make a balance check to stay there.
I think DC 19 sounds about right (2-6 inches wide, if you pick a bigger limb, but sloped a bit and flexible, which we'll call "slippery").