Sephiroth's will is certainly no joke.
It's even stronger than the planet's will(Lifestream/Minerva) during VII.
Even though in FFVII he loses to Cloud in a battle of wills, in AC he becomes absolute again in AC(probabl becasue he is now literally akin to a god like Minerva because of his own Lifestream).
My guess is his existential force has always been stronger, like Cloud.
They both go on to talk about how they were always different from the other kids, how they were special, etc...