It can never survive bullet punch in the first place.
252 SpA Mega Gardevoir Hidden Power Fire vs. 252 HP / 4 SpD Mega Scizor: 316-372 (91.8 - 108.1%) -- 50% chance to OHKO
252+ Atk Technician Mega Scizor Bullet Punch vs. 4 HP / 0 Def Mega Gardevoir: 338-402 (121.5 - 144.6%) -- guaranteed OHKO
Edit: even with a 252 hp/252 def bold Gardevoir, Hidden Power won't OHKO, and Bullet Punch in return 2HKOs. You can only kill Scizor if you have 252/252+ def and hit him on the switch.
what if it had 4 ev's invested to def. and reflect was up?