Current going theory for damage formula, based on Persona 3's setup, may change later:
sqrt(((@a.str * 8) / (@b.res * 4 + @b.def)) * power) * 7.4made with @nex3's syntax highlighter
Just gotta determine if the growth rate of this damage works out as stats increase, whether to include level difference scaling (and how to level enemies), and if this feels good for weapon and other damage types.
This would also suggest that defence is divided during damage calculation, but not against calculated power like the Megaten wiki believes.