Mew is seriously a beast and needs to be used more often. BP sets are SO meh. But part of Mew's unpredictability (I think), is that everyone always first suspects a BP set, especially if it's on an offensive team, which means the opponent usually switches in their hardest hitter or a pokemon to stat up along side it.
I've been using a the following set. While I came up with it myself, I'm pretty sure it's been thought of before.
Timid @ Leftovers
252 HP/4 SpDef/252 Spd
Will-o-Wisp
Taunt
Recover
Psychic
And it's the shit. Takes 100% max from a modest specs Latios's Draco Meteor, meaning if I'm feeling ballsy, I can recover off the DM damage while Latios digs it's own hole with the -2 stat drops. Trick sucks though.
After a burn, practically nothing on the physical side can get through it. Outrages from the likes of Dragonite and Haxorous barely do 50%. The only stuff doing over 50% are CB T-tar crunch and CB Shubarugo Megahorn, and even those only do around 65&. Psychic lets it smack around pokemon trying to get a guts boost, revenge kill protect blaziken. (but not SD variants) and other fighting type pokemon that give my team a hard time.
The one thing I love about this set is that it dismantles both offensive teams and defensive teams with equal ease. The only answer stall has is Poison Heal Gliscor and Heatran. Any other wall is demolished by taunt + will-o-wisp. Given the physical inclination of most offensive teams, this Mew's burns are honestly a godsend, being able to lure in X-scissor terakion and most dragons. It falters a bit against weather teams but can still pull it's weight.