@lu1z:
First off. It's nice to see you are a One Piece Fan. OK. Priority #1. Done.
Second. Toxicroak's most dominant set this metagame in my opinion, is the devasting LO Swords Dance set with SD, Drain Punch/Cross Chop, Sucker Punch, and Ice Punch.
Hopefully you are using that set.
You obviously know that Politoed is a given as they have fantastic synergy for two frogs of a pond. Grass (mainly, Celebi and Ferrothorn) are the ones to switch into a Toxicroak. Unless Celebi carries Psychic, both Grass 'Mons are easily eliminated through the appropriate moves. Politoed induces Rain, yet it fails to take heavy hits from the likes of Keldeo. Toxicroak sucks up Hydro Pumps and can eat Secret Swords as it sets up an SD. Drain Punch should OHKO at +2 and give you back all that delicious HP.
So far we have Politoed and Toxicroak
You say you struggle with Jirachi. Sp.Def Jirachi sounds more like your problem. For those cases, Landorus-T should do the job. Lando-T not only scares Jirachi, but it sucks up nasty Electric Type moves (careful though. BoltBeam.) and acts as a physically defensive pivot while setting up Rocks. Scarf Jirachi can be dealt with by using Ferrothorn. Ferrothorn sets up hazards, can use Leech Seed (it's up to you), can tank hits and hurt Jirachi with Iron Barbs as it gets locked, soaks up Electric Type moves, and is Grass. You could potentially get some sorta FireWaterGrass FWG Core going on if you want.
Try out Ferrothorn or Lando-T depending on your needs. With these three Pokemon you should get a solid balanced core.
Offense: Toxicroak
Specially Defensive Pivot: Politoed
Physically Defensive Pivot: Ferrothorn/Landorus-T