The Xatu Warrior
Ash greninja, like all in battle form changes, will revert to the base form before megaing. This is to stay consistant with ingame behavior, as Mega-ing a mega pokemon (such as having an mmy with mmxite) will add the stats to the base form rather than allowing some double mega.
if a coder can prove to me that mega evolving forms reverts them to base is true/provide evidence my facts are wrong, id appreciate it. sorry for making a big deal out of something moderately small, but i do enjoy my mechanics mumbo jumbo, and this is a significant aspect of the meta that should be corrected if evidence against the mechanic exists(at least until someone smarter and knows more about usum's coding comes and debunks my thoughts :P).