My strategy is:
get one Doubleteam up at Gengar (which takes some tries because it has Nightshade/Confuseray and is faster)
get Substitute up
spam Doubleteam to get +6 evasion
somehow be lucky that Agatha doesn't switch to Golbat and uses Haze, which removes all stat-changes
spam all x-items (preferably on Agatha's first Gengar) for reapplying the badge-bonus.
(using a Dire Hit to avoid critical hits would be also a good idea!)
- use Psychic
Now in retrospect I should have brought elixirs with me, but it worked out anyway.
But is there a better way, maybe forcing Golbat to switch(and not be hazed) with some trick?
Right now it's a luck-based strategy and I would like to know if there are any tricks, such that the luck part is reduced to a minimum.
Off the top of my hat I see only RNG-manip and I don't know if it's possible that Agatha always uses moves that don't harm me (no Haze would be great!).