Not true DrJo. Destiny Bond has no priority, and Gengar is deathly afraid of Scizor. So I can step in and bullet punch before you get Destiny Bond off. Plus, who says Gengar gets a kill before it tries to DB? No kills are guaranteed dude. Unless you are using a wobbuffet.
But I digress, Wob comes in, Encores the move you use. If its Physical, he counters for massive damage. If its Special, he Mirror Coats for massive damage. Don't think you can just stall out the Encore with non-attacking moves, because if you lock yourself into a non-attacking move, he switches out and something sets up on you and rips holes through your team. Or instead of sending someone to set up, they send in someone Wish-pass to Wobbuffet, keeping him healthy.
Don't think you can avoid all of this by switching out. Shadow-Tag prevents that. Don't think its easy to sidestep getting your attacks bounced by OHKO him. His HP is phenominal. Don't think you can Toxic stall him. If he suspects any funny business he'll Safeguard up. Or he doesn't run Safeguard, he runs Destiny Bond, probably with a Wish + Aromantherapy Blissey to keep him happy.
Counter and Mirror Coat don't go off of percentages. They go off of actual Hp numbers. So if you do 175 damage to him (Im guessing that about 1/3rd his health), he will respond with a 350 damage attack, which is not something that most sweepers can handle. So ya... Guaranteed kills = Uber status