Someone suggested taking a high armour shield and approaching ranged backwards, but Paul is right in that it would be a really bad idea
Don't knock it until you try it. Sometimes I put my shield on my back and just stand on a wall with my back facing enemy archers. The angles presented by siege maps mean I am pretty much 100% covered, even though my shield is pretty small (knightly heater).
Projectiles that hit my shield are mitigated by 109 armor, causing no damage to me or my shield but surely causing enemy archers to rage a fit.