I'm not so sure about straight-up Night March being that good with the "XY-On" format. Losing Mew EX will be a huge blow to NM since that's what gives the deck speed.
Joltik will be the quickest way to attack without Mew but it only has, let's be ready, 30 FRIGGIN HP. Isn't that alot? Yes it is enough. Only some people from XY-On can one-shot in a turn; baby Yveltal, Seismitoad EX and best of all, if Latios EX goes first, he can one shot Joltik!
Obviously the NM player can be smart and only bench Joltik. But it's not safe from Escape Rope, Lysandre (that's a bit of a stretch), Metagross from AOR and bench damage. You don't have to worry about a Hammerhead, but you can still be sniped from M Latios, Manectric EX, Hoopa EX (if someone is insane enough to power it up). If Noivern ever does become a thing, you can also OHKO benched Joltiks and 2HKO Pumpkaboos. Echolocation may also prevent the 220 damage from an annoying low HP Pokemon that can OHKO everything except for Wailord.