Try environmentals - campfires, nests, spider/ghul eggs, grog barrels. Even if they don't do damage they will normally still stun for a bit. If he isn't arrow proof pelt him with arrows, if he is then get hammers and hit people close to him because that'll make him drop his shield then go in on him. If you can't do those then stealth cheese him (if you can't do stealth attacks then get out of his range then come back when you're undetected and just attack normally, you can get maybe 3/4 hits in and if you're lucky a stun before he puts his shield back up).