Synthisizing Arumats Conqueror Scythe "Vajra"?

I wasted my laser weapon so i had to use the conqueror scythe, what factors should i give it i'm in the wandering dungeon trying to get the best equips for the ethereal queen battle i have max attack stats. some guides tell me to give it hp damage +20% factors but i never use arumats regular attacks i only use his dragon roar attack for aoe what factors should l i use for him? i'm thinking the following:

+20% dmg to humanoids (compulsory)
increase crit chance
add stun to attack
mp cost -10% in battle

i never have mp problems in battle so the mp cost might be a waste, i noticed an mp damage +20% factor, what does that do? do that affect dragon roars damage?

if you could suggest a synth set up i'd appreciate it thanks.