Mega Ascension lets you search your deck for a Mega Rayquaza and put in your hand. That being said, Mega Ascension is a reliable way to mega evolve Rayquaza.
However, it is very much possible to mega evolve it without this move. You may happen to draw one from your deck or your Prize Cards. There are also other trainers and moves that allow you to search your deck for a Pokémon. To mega evolve it, you place Mega Rayquaza-EX on top of Raqyquaza-EX, just like you do normal evolutions.
Hope I helped! :)