ger's description: a stand power that manifested when a stand was pierced by the arrow the strengh of attacker's will be reverted to zero those who are struck by this power will have even their deaths reverted to zero. and thus coninously die an infinite number of times
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basicly this means that ger can only rtz attacks, or traduced: unless you attack ger, he won't rtz you.
so itachi could make shadow clones, activate mangekyo, or use a genjutsu on giorno (neither gold experience or requiem has never shown any kind of defenses against ilusions or mind control),but he couldn't use amateratsu since is is a technique which requiem stomps,since both are great at strategy,so they would start looking for weaknesses, giorno's only weakness is that he can't nullify nothing which isn't an attack, itachi's is his resistance. since itachi has way more techniques, including shadow clones which pass you their experience when they vanish,and he is also way smarter and experienced than giorno, he would understand ger and discover giorno's weakness before giorno discovers his, at this point, itachi has at least 2 options to easy win:
-tsukuyomi giorno untill he is mad, without harming him in the ilussion (he could traumatize giorno making him think that he killed every loved by him, and leaving him alone 30000000 years, but not harming him like he did with kakashi) cause it could make ger activates
-use izanami which is a perfect counter to ger