Accoring to HH website, Kawn's shield multiplier is 3.5*Def, so let's say you give him build of 6K Def & 50K HP.
His A3 will put up a shield worth 21K, which is 42% of 50K, so his A2 would ignore 21% of Enemy Defense on top of the Savage/Lethal/Merciless Gear and Helmsmasher that you would want on him too.
HH does rate his A2 as "Godlike" for Arena, but that is predicated on the correct setup, which for Kawn means a shield is up before he hits.
Problem is are you getting 2 turns like that in arena, and if you are why bother with the extra step when so many other options are just better. Of course you could run him with Bolster Nehkret or something to that effect but Narses exists.