Ferrothorn is almost always a better option than Roserade, both as a Spiker and Fairy resist. Azumarill is a good option for a teammate, yes, as it takes on Choice Scarf Latios and Heatran very well, both of which Mega Sceptile hates. Sceptile can also have a very hard time Mega Evolving becuase of all the faster Pokemon out there. Thus, having some Speed control is favorable. Thundurus is the overall best option, with priority Thunder Wave. It is also great for taking on Mega Salamence, which is probably the biggest threat to offense. As for the rest Pokemon, the choice is entierly up to you; what are you confortable with? Greninja can be a great partner, as it and Mega Sceptile draw is some of the same Pokemon. Choice Scarf Landorus-T is another good option, as it deals with opposing Choice Scarf Landorus-T, and some physical attackers like Mega Beedrill, Mega Gallade, and Mega Lopunny.