Use Chandelure. It doesn't take up the mega slot, and it pairs pretty well with Salamence. Salamences main weakness is Ice, and Chandelure can easily switch in to it. Mega Alakazam is really weak to physical moves while Mega Salamence is bulkier. The only downside is that you stack a Rock weakness with Chandelure, so your third member should probably be a rock resist.
Hope this helped!