set /a "hp = hp - ((atk*arandom)/((edef*6)/edefbonus))"

set /a "hp = 50 - ((2*2)/((1*6)/8))"

PS Y:> $hp = 50 - ((2*2)/((1*6)/8))
PS Y:> $hp
44.6666666666667

set /a "hp = 50 - ((2*2)/((1*6)/8))"

SET Whole = Result / 100
SET "Fraction = Result %% 100"
SET Result=%Whole%.%Fraction%

10*2*2*8 =320
1*6      =6
320/6    =53
53+5     =58
58/10    =5

2*2*8 =32
1*6   =6
32/6  =5   (we just lost the fraction digit, so rounding won't work)
5*10  =50
50+5  =55
55/10 =5