Hand-held shields weren't a huge thing in Sengoku era Japan, but large deployable 'shields' called pavices were used extensively by infantry. This might be tough to impliment, but I've seen at least one mod that gets it to work.

http://forums.taleworlds.com/index.php?topic=117391.0

https://www.youtube.com/watch?v=Olnq32yEmZQ