He/she has to be fully promoted before learning expert disarm. I got caught the same way, the first time I played. You need to equip him/her with an item of disarming, the higher quality the better. You will need more points in disarm anyway, but it does work in the end.There are two ways of finding your item of disarm. The first way is to depend on luck - sometimes it works, sometimes it doesn't. The other way is by mephistoing one of the item shops. Go to a shop you've not used before, save your game outside, and then enter and see what he's got. If you don't like the selection, reload your savegame and try again.
FYI, this technique took its name from Mephisto, who was among the first to report it.
HTH! 
