Or at least how much defense affects the end result?

Basicly

Z ¤ (Brother's POW - Enemy DEF / 2)

where Z is a constant that depends on which attack is used. http://tasvideos.org/GameResources/GBx/MarioAndLuigiSuperstarSaga.html#AttackAlgorithm http://tasvideos.org/forum - you can ask jdaster64 via pm there There is also this formula he posted:

((BrotherPow ¤ BrotherPowMod ¤ 0.4) - (EnemyDef ¤ EnemyDefMod ¤ 0.2)) ¤ AttackMult

I have yet to implement damage prediction etc. to my luascript. Have not been feeling like working on it lately.

Assumingly for monsters it'd be

Z ¤ (monster pow - bro Def / 2)?

It probably works like that, both ways, yeah. But I can only give you this rough information. If you want more details you can ask jdaster64