I did it anyway. Go me!

Code:

@echo off
setlocal
set /p stat="Stat to calculate (HP, Atk, Def, Spd, SpAtk, SpDef): "
echo.
set /p base="Base stat: "
set /p iv="IV: "
set /p ev="EV: "
set /p lv="Level: "
set /p nat="Nature: "
set natval=10
if "%stat%"=="Atk" (
if "%nat%"=="Lonely" set natval=11
if "%nat%"=="Brave" set natval=11
if "%nat%"=="Adamant" set natval=11
if "%nat%"=="Naughty" set natval=11
if "%nat%"=="Bold" set natval=9
if "%nat%"=="Timid" set natval=9
if "%nat%"=="Modest" set natval=9
if "%nat%"=="Calm" set natval=9
)
if "%stat%"=="Def" (
if "%nat%"=="Bold" set natval=11
if "%nat%"=="Relaxed" set natval=11
if "%nat%"=="Impish" set natval=11
if "%nat%"=="Lax" set natval=11
if "%nat%"=="Lonely" set natval=9
if "%nat%"=="Hasty" set natval=9
if "%nat%"=="Mild" set natval=9
if "%nat%"=="Gentle" set natval=9
)
if "%stat%"=="Spd" (
if "%nat%"=="Timid" set natval=11
if "%nat%"=="Hasty" set natval=11
if "%nat%"=="Jolly" set natval=11
if "%nat%"=="Naive" set natval=11
if "%nat%"=="Brave" set natval=9
if "%nat%"=="Relaxed" set natval=9
if "%nat%"=="Quiet" set natval=9
if "%nat%"=="Sassy" set natval=9
)
if "%stat%"=="SpAtk" (
if "%nat%"=="Modest" set natval=11
if "%nat%"=="Mild" set natval=11
if "%nat%"=="Quiet" set natval=11
if "%nat%"=="Rash" set natval=11
if "%nat%"=="Adamant" set natval=9
if "%nat%"=="Impish" set natval=9
if "%nat%"=="Jolly" set natval=9
if "%nat%"=="Careful" set natval=9
)
if "%stat%"=="SpDef" (
if "%nat%"=="Calm" set natval=11
if "%nat%"=="Gentle" set natval=11
if "%nat%"=="Sassy" set natval=11
if "%nat%"=="Careful" set natval=11
if "%nat%"=="Naughty" set natval=9
if "%nat%"=="Lax" set natval=9
if "%nat%"=="Naive" set natval=9
if "%nat%"=="Rash" set natval=9
)
if "%stat%"=="HP" set /a val=((2*%base%+%iv%+%ev%/4+100)*%lv%)/100+10
if not "%stat%"=="HP" set /a val=(((2*%base%+%iv%+%ev%/4)*%lv%)/100+5)*%natval%/10
echo.
echo %val%
pause
endlocal

Copy and paste into Notepad, save as a .cmd file, open it to run it Remember to use capital letters at the start of words ^_^