The roughly translation of the skill effect from WN was “when you get 0 damage from close range attack, it will return damage equal to skill holder’s defense.” His initial defense stat was 3+5+10 from stat + equipment + passive skill. So it’s OHKO. Also the normal damage calculation is (attack - defense)/2 so that level 2 monster didn’t have attack of more than 18.
I know I'm reading this for free and I shouldn't complain since I also don't want to go and do this kind of stuff myself, but I see it so often in stories like these where they have status windows and they are always wrong with what the stats should be. literally just a one page difference between it saying he gained 10 DEF and showing the status screen where it only show the original 3 then +5 which I'm guessing is from his armor that doesn't look close to beginning armor. where did the 10 go? it just doesn't bode well for accurate stats later on or it'll end up being wrong in other ways. idk just sucky to see a lot and this is the one that caught the comment.
