Parting Shot, switch into a Shadow Tag/Arena Trap user and you could score some nifty KOs. Scarfed Base 58 Speed will outspeed unboosted threats, and it is just about Bulky enough to survive the hit of an unboosted hit from a sweeper. Ubers especially where the entire tier is pretty slow, and with Sticky Web support if you're feeling very reliant on a Mega Gengar sweep can be dangerous.
Weirdly enough it's outclassed by Smeargle somewhat in this respect, who can beat almost every unboosted attacker with its good enough Base 70 speed. Against stuff like Groudon, which is slow but powerful, Parting Shot it, switch to regular Gengar, Mega evolve, absorb the Fire Punch then either set up a sub if you're feeling risky or just remove it and then get your Kyogre to do whatever.
That being said, if its only going to lower stats once or twice, why not use a faster Memento user like +1 Memento off a Whimsicott? Does its Base 124 Attack come into play here? I feel some testing is needed