Why this mini game on hard level is impossible to do? Over 30 attempts and none close winning -.- I don't even want to think what will be on hard mode + Imo for s mega gengar costume it's too much. 

So you are stuck at the hard mode or at the "very" hard mode where you can get the shiny costume?
I can tell you thats not impossible but I could only give you advice at the hard mode,

@BashI'm stuck in hard mode (free iv rr). I'll beat this mode to try hard mode + and get shiny costume. I'm watching/reading some tips for hard mode but it's impossible for me. If You'll give me advice i'll appreciate it

First check if you have 5 Pokémon aviable, if not its really impossible. You would have to fix that by redoing the old quest.

But expectign you have 5 mons, I made it this way:

 

See a Golurk? -> Give up. Its easier without it.

No Golurk? -> Always Flareblitz Weavile with Arcanine in 1st round if present -> Always Moonblast Scraggy with Clefable in 1st roundif present

-> Always try to hypnose Trevenant with Munna in first round if present

 

Try to Rockslide opponent "packs" with Cubone, means when there are 3 opponents in one field, a Rockslide is very nice because of the high flinch rate.

After the first round, try to defeat squishy Pokémon first.

Make sure there is always a Pokémon on the Spatk and Def field.

I kept Arcanine and Rattata alive in order to to Flare Blitz and Crunch the boss Gengar

 

Dont hesitate to give up the round if its looking bad.

i have watched belzebel viedeo on the minigame and i made it after 15 trys 😄

I asked the same.

 

https://pokemonrevolution.net/forum/topic/162162-how-you-win-in-hard-mode-halloween-quest/?tab=comments#comment-901499

It's impossible to do. I fight with gengar 6 times, he always kill my pokes in 1 hit even when i use vitamins. He always faster and stronger. **** this gengar.

If he oneshots Arcanine and/or Rattata, he got a SpAtk boost. You need to avoid that!
He gets all boosts from the Pokemon you defeated. You can prevent that by holding the field which gives a SpAtk boost (You should hold the one with a Def boost as well).

Like Bash already mentioned, you have to always deny the SPATK and DEF boosts to the pokemon an the battlefield as all boosts those pokemon collect will be added to the Mega-Gengar in the end. Thats the most important thing to keep in mind.

You do that by always having one if your pokemon on the SPATK and DEF boosting fields. If the opposing pokemon collected one of these boosts you are better off to surrender and start again as you wont be able to beat the Mega-Gengar in the end anyway.

It doesn't matter anymore, playing on holding some fields is not for me. I can barely defeat these pokemons, I have no patience and nerves for it. I have to accept the lack of s mega gengar clothes. Thanks for the advice and sorry for wasting your time.