>>16124882I haven't use it myself, but here's what I imagine happens:
>Turn 1Gengar switches in on something it can kill
>Turn 2Foe switches to a Gengar counter/wall. Gengar Mega Evolves and uses Perish Song.
Perish count: 3
>Turn 3Mega Gengar uses Protect.
Any attempt at retaliation fails.
Perish count: 2
>Turn 4Mega Gengar uses Substitute, avoiding an OHKO. If the foe has priority or will outspeed and kill, then Protect has a 50% chance to work again. Granted, you should only be trying to kill slower things without priority here, preferably walls like Chansey.
Perish count: 1
>Turn 5Mega Gengar switches out. Because of how Shadow Tag works, the opponent remains trapped that turn.
Perish count: 0
The target faints, and you have a Mega Gengar that can come in and do the same thing up to 3 more times. If Mega Gengar switches out after Mega Evolving because the foe's switch-in would beat it, it can still come in later on something that it can always Perish Kill because Shadow Tag is no longer delayed.
In the end, MGengar is no different from a long OHKO move. It doesn't require prediction, and it can't be stopped unless you specifically build your team against it.
