The seekers are resistent to piercing and slashing damage, but not to blunt. Also they have a weak spot on the back (useful for sneaky attacks or when is a dwarf). The giant fliying tick has a weak spot on the bottom, so the arbalest is great to deal with them.

That said, I loooove the new Atgeir, the lightning effect is so cool!!. I always use Atgeir, so I like this pretty much.