Gold enchant stone with stun chance, high lvl ultimate accessory (the ones that gives strength also increase stun rate), advanced partner mod modification: you can replace all 7 mods for stun chance
in addition, been said/reminded several times, but still better to repeat:
you can't create new skill from mod or enchant!!!
i.e. if a partner doesn't have a stun skill, no matter what rate you will add, 0% +3000% is still 0%...