Level up Shroomish until level 45. He will learn Spore and then try to evolve. Congratulations, now you have a Sporing Breloom.
Now, you go to the Move Relearner (Pastoria City, just look for the house on the southeast corner of the city), give him a Heart Scale and let him teach Mach Punch for Breloom. And now, you have a Sporing Breloom with Mach Punch.
And for heaven's sake, just really use Breloom with Poison Heal + Toxic Orb. Effect Spore is bad for anything, even Parasect.