I've already got two Megas on my team, Mega Mewtwo is out. Considering the the environment I'm expecting, I've got two items for consideration: Colbur Berry and Assault Vest. The former allows Mewtwo to survive a Sucker Punch or even a Dark Pulse from Yveltal and deal some hefty damage in return. It will still likely fall before taking it out, though. The latter gives it a solid chance to survive a Power Herb Xerneas' Moonblast after Geomancy, and potentially 2HKO it. Here are some calculations:
+2 252 SpA Fairy Aura Xerneas Moonblast vs. 0 HP / 4 SpD Assault Vest Mewtwo: 303-357 (85.8 - 101.1%) -- 6.3% chance to OHKO 252 SpA Mewtwo Psystrike vs. 4 HP / 0 Def Xerneas: 195-229 (49.4 - 58.1%) -- 98.4% chance to 2HKO
This means that if they're sent out on the same turn, Xern with use a turn to Geo, tanking a hit. It will then Blast Mewtwo, who has a solid chance for survival and KO it back with high a likelihood.
252 SpA Life Orb Dark Aura Yveltal Dark Pulse vs. 0 HP / 4 SpD Colbur Berry Mewtwo: 251-296 (71.9 - 84.8%) -- guaranteed 2HKO 4 Atk Life Orb Dark Aura Yveltal Sucker Punch vs. 0 HP / 0 Def Colbur Berry Mewtwo: 208-246 (59.5 - 70.4%) -- guaranteed 2HKO 252 SpA Mewtwo Thunderbolt vs. 0 HP / 0 SpD Yveltal: 226-268 (57.5 - 68.1%) -- guaranteed 2HKO
This shows that one on one, Yveltal will still win (unless it lacks Sucker Punch for some reason), but Mewtwo can take off a big chunk of health before going down.
I'm running a Mega Scizor and a Sucker Punch Yveltal myself (albeit of a hindering nature), so I have revenge killing possibilities. Now...I'm honestly expecting Xerneas to be the more common threat. I do have my Mega Scizor to help with it, but once it gets rolling it can really screw you over. I don't have a lot to help with Yveltal, though. Colbur pretty much ONLY works for Yveltal and MegaMawile, while AssVest has more utility.
Can I get some input on this? --- 3DS FC: 3050 - 7604 - 7991 Friend Safari: Octillery, Floatzel, Frogadier
I'm still leaning towards the AssVest because of fear of Xern. I don't expect as many Yveltals, but it's also a significant threat - one I lack decisive means to counter. For the record, the team I've planned is: - Mewtwo - Yveltal (Life Orb) - Megazard Y - Mega Scizor - Sableye (Leftovers) - Gyarados (Intimidate; Lum Berry)
Being limited to Kalos pokes and not wanting to succumb to Talonflame/Aegislash/Rotom doesn't provide many options :/ --- 3DS FC: 3050 - 7604 - 7991 Friend Safari: Octillery, Floatzel, Frogadier