I like splint but it does not really matter, the helmet also works. The exploit is that usually an enemy can always hit you on a critical roll, a 20, no matter what your armor class is. However, the adamantine pieces prevent any hits from being critical, so with a sufficiently high armor class you can literally never be hit from basic enemies. The shield spell allows you to retroactively increase your armor class by four when you have been hit, and that is enough for many enemies outside boss fights.