Are you asking if its possible to earn enough points in the arena to get Omnislash if Cloud's level is 8?

Or are you asking if its possible for Cloud to be able to use Omnislash at Lv. 8?

For the first question, yes, though it would take forever since Cloud wouldn't be able to survive really hard encounters. It would be terribly time consuming but I think its possible.

If its the second question, then I would say its impossible without a hack or being the one little event at the end of the game. Cloud cannot use Omnislash normally without acquiring all his previous Limit Break levels. The fact is, just meeting the requirements for those levels will surely level Cloud beyond Lv.8.

I hope these answer your question(s).