Hm, Shishio's pretty good physically and reasonably fast, and the 'no falling for the same move twice' thing is nice, but he's very straitforward by Naruto standards having no genjutsu or ninjutsu, only being a swordperson/taijutsuist plus some minor tricks (the flaming sword and exploding glove are pretty minor next to the stuff the ninja pull). We never really saw his third secret sword move due to the combustion thing. No higher than chunin.
Haven't read enough Berserk to rate Berserker Armor Guts.