There we have the crux of the problem, at the moment it's sheer weight of numbers trying to attack a base essentially only using the x,z axis. This is what makes stilt bases so effective and why I generally avoid making them. Sure, with no other option they should lash out at whatever they can hit but should targeting structural weaknesses be their first course of action?
Three options for dealing with stilt bases have been tried, spider zombies, aerial threat, and gore blocks. Spiders can't climb a greater angle than vertical which makes them trivial to circumvent even without a stilt base. Vultures, and their predecessor bees, are more of an annoyance than a threat. Gore blocks were dropped due to the amount of work required to get them working properly.
Don't get me started on the need for a decent threat to underground bases.