The "I was let off with a mortal injury" line is originally a meme expression from forex trading (and stock trading) discussions, which basically means "to suffer very heavy, financial-crippling losses (but at least one's life isn't totally ruined yet)".
This then got transplanted into action manga and action video games where the "mortal injury" is usually just exaggeration for "serious injury" or "heavy damage", not quite actual "mortal" injuries. It sounds kinda weird and oxymoronic if examined too deeply, but since it's already a set expression of sorts most people just let it fly.
Updated
Ughk......ggnnk.
PRRRREEEECIOUS!
That was a close one..
Fehhh...
Haahhh...
Mortally wounded but alive... somehow.Original is an forex trading meme (lit. "I was let off with a mortal injury", i.e. "heavy losses") that found its way into action fighting manga.