Speaking of Mew, Mew is a great recipient of a Shell Smash pass then as Mew can continue the chain if need be with Baton Pass, add to it, and attack on both sides (although special is way superior) or be the one providing the Screens...that is something Deoxys can't do, have the option of either screening or chaining or both. Psychic is not a terrible stab (especially for Mew who has all sorts of things to mess with steel types) and hitting the many fighters this gen is essential as is resisting their stab. Doing it with great stats everywhere (god, imagine if it had Magic Guard as a Dream World or something, argh so many potential abilities lost, darn Game Freak for not giving a second option ability) with reliable recovery and speed is awesome. Mew has the bulk to take a lot of those hits because of Roost (with Hp investment although even with Min def Mew could take Terakion, Landlos, Doryuzu hits and survive, and unboosted even afford to Will-O-Wisp and or Roost later). Draco Meteor is a pain but it can survive it with hp and sdef investment, laugh at Latios as it Roosts, and sets up Screens or Will-O-Wisp on the next things in. Poison/Burn/Para is annoying but you get to reflect the status at least once which could help and Mew is more of a bulky hit taker.
The difference is that Mew is a bulky Nasty Plotter while Thundurus is more frail but a set of Nasty Plot/Psychic/Aura Sphere/Roost (or even Calm Mind which would make Mew really painfully hard to kill) would hurt much like Rankurusu's would except Aura Sphere is reliable, Mew's attack is much stronger than the max hp/max def Rank's after Nasty Plot, it has good defenses, and Psychic hurts almost everything but Steels and other Psychics. It is still reccomend to have something to take bulky steels out but Mew is not outclassed as an attacker, Nasty Plotter or otherwise because Mew survives some ridiculous things and the fact is that it has so many sets to run until you see what it does, the opponent still has no idea if it's a Screener, a bulky attacker, a statuser, a phazer, a weather set upper (although if it's on a weather team, you'd think that'd be a dead giveaway although it would lure in their Tyranitar for you to destroy...), a Scarf Transformer, and on.
The things that threaten Mew look more like these: Shubargo (no Flamethrower although it won't enjoy burn as much), Scizor (same as Shubargo), Latios (Draco Meteor), Sazandora (Draco Meteor and/or Dark Pulse), Sableye (Mischievious Heart Taunt, Mew simply can't do anything long term to this guy really), Spiritomb (hah, like anyone uses it but if you do), other Psychics (without Shadow Ball/carrying Light Screen, especially if they carry Shadow Ball), bulky Taunters that don't die to Mew, etc. Physical hitters like Landlos, Terakion, Doryuzu are secondary (unless they got Swords Dance but one is assuming they come in on Mew, not the other way around) and are handled by things like Icebeam, Will-O-Wisp, Reflect, Psychic, Trick Room, etc in Mew's arsenal. Terakion does have a weakness to Psychic so if you're dumb and you get burned or Mew get's Nasty Plot, Mew can 1-2 hit ko you with that. I don't know why that was mentioned.