I heard that in the Omega Ruins you can get 99x Warp spheres if you can open the 12 chests without fighting a monster
Any ideas how i can make it happen?

Sounds false... but why would anyone want 99 Warp Spheres, I have no idea. :(

Yeah, I really believe that this is just a rumor as well. And, to answer the question:

It's random which treasure chest is filled with the monster. So, there is no definite order in which you have to open them up.

Yeah the 99 warp Spheres is true its on a FAQ on gamefaqs i havent managed t do it but it would be worth it.