Yea, his armor is basically a psuedo-living medieval Iron Maiden torture device. Imagine hundreds if not thousands of iron needles piercing through all your muscle tissue into your bones. Imagine it doing that multiple times. Yea, Guts has always had tons of scars but I can only imagine the damage his body is taking every time he unleashes the armor.
As for Caska well, he went through hell for her... litterly. He's starved himself, watched, protected tolerated and sacrificed everything for her. What do you think she is to him?