I was pretty sure, as a kind of noob I had no chance to defeat Pumpking but I tried anyway... and won!!! Thanks to tips from the guide and players (and some items + lots of luck)
It was an epic and really fun fight (for me)
[spoiler]So I quickly put together a random Team: Dragonite (shell bell), Zangoose (quick claw), Persian (lax incense), Manectric (lum berry), Aerodactyl (blackglasses), Jolteon (magnet) - all Lv.100 except zang
1st stage: I used my dragonite, with 2x dragon dance and some fire punches, easy win
2nd: dragonite got killed and I used my joker zangoose (randomly found it in my pc, used because it had taunt), and luckily attacked first with taunt
but my plan had flaws, zangoose had 3 useless normal-type attacks and I couldn't switch, damnit
then I don't know exactly what happened after, but because pumpking used roar a lot, I got to use all my pokemon and each had the chance to make some damage,
I used mostly flamethrower (manectric) thunder (manectric/jolteon) shadow ball (jolteon/persian) and killed him
3rd: So I heard that you should use toxic, but I forgot to bring the pokemon who had it ... great
ok at beginning some of my pokemon got killed by superpower, and my persian used screech, so the def decreased a lot and I had the chance to weaken it with aerodactyl's crunch, before he used harvest
then I attacked and attacked without much effect and was close to lose, but I used about 5-6 max revives that helped me live longer
so then I managed to burn him with dragonite's fire punch, and I had luck that he was too dumb to use harvest again, and he always jumped up, so he kept losing hp because of the burn
I only have the end text of the battle:
YAY! =D[/spoiler]