I am just going on what I've read.
According to what I have read, Sephiroth was destroying solar systems, not necessarily the one that they were on.
Any villain is stronger than the hero, and the hero usually gets lucky. In most a Final Fantasy games, the hero is accompanied by a group. Cloud took Sephiroth, the one who is capable of destroying solar systems, and beat him.
Cloud was chosen because he MANAGED to beat Sephiroth, but he himself does not have the capability of destroying solar systems, which puts him much lower in the list of tiers.
According to this information, Sephiroth having this amazing capability, he could destroy Cloud and Kuja in one blow. If Kuja struck preemptively, he could kill Sephiroth and Cloud in one blow, because I doubt they could breathe in outer space even if they did manage to survive, and because Cloud MANAGED to beat Sephiroth, without the help of his friends, that automatically puts him in the #1 spot.