I find that NPC trainer can mega evolve multiple pixelmons in a battle.
I set Rayquaza, Tyranitar, Blaziken, Diancie in one NPC trainer and he evolved them all!
When I tried to escape the battle he then mega evolved one pixelmon and let me go.
The situation happened in both server and client.
What's more, when I escaped from this kind of battle in servers, it caused a crash. Following are the crash log in pastebin:
https://pastebin.com/tV2eXFLkAdditionally, I hope that a NPC who has mega pixelmons will have an option of equip a mega ring, so it looks reasonable to mega evolve his/her pixelmons.
Thanks for dealing with so much issues :)