The MND wont increase the Flat Blade/Spirits Within Multipliers. @Lvl 50 and ~240 MND they are already capped.
But in saying that, STR/MND greatly increases the damage from both
~50% more damage @ around 320 STR/300 MND (up from 232 STR/234 MND) Which is much greater than HP alone.
This is a decent fight for this type of build because Miser is so squishy - and the Divine Veil buff works wonders for the Monks nearby.