aah that's true, as long as you switch in on the SD.Mega Gyarados does not really care much about Mimikyu.
It does not get OHKOed by a boosted Play Rough and it can, in fact, Dragon Dance freely on it.
+1 252 Atk Mimikyu Play Rough vs. 0 HP / 4 Def Gyarados: 208-246 (62.8 - 74.3%) -- guaranteed 2HKO
Then, it's a matter of mega evolving, as you now outspeed Mimikyu and Shadow Sneak does laughable damage.
+1 252 Atk Mimikyu Shadow Sneak vs. 0 HP / 4 Def Gyarados-Mega: 36-42 (10.8 - 12.6%) -- possible 8HKO
A life orb set still poses an issue though.
+1 252 Atk Life Orb Mimikyu Play Rough vs. 0 HP / 4 Def Gyarados: 270-320 (81.5 - 96.6%) -- guaranteed 2HKO
+1 252 Atk Life Orb Mimikyu Play Rough vs. 0 HP / 4 Def Gyarados: 270-320 (81.5 - 96.6%) -- guaranteed OHKO after Stealth Rock