First mistake you made is buying items before having full army. What good is great weapon, if there is no one to benefit from it ?
First and foremost is securing you win battle, then worry about bonuses for army and medals/full maan-rage pool and such.
Second out of battle what does Digger rage skill says ? If it`s 4 chests/2shrines and in battle you have only 3 chests then move around your units, it`s possible that some of your stacks are standing on chest green spot. And do quick dig check in first turn before any units move to see how many green/red spot you see.
Third ghost sword doesnt give enemy units -50% physical res , but 50% of their basic value. So for your example assassins have basic physical res 0%, remove half of it 50% = half of 0 is still 0 so they stay on 0%.
But if you attack lets say horseman who have basic phy res 20% you`ll lower it to 10%.
Ghost sword is total crap item, i`m sorry to say you wasted gold on it
