Breloom can only learn Bullet Seed by breeding.
First, catch a MALE Sunflora, Mantine, Exeggute, Hoppip, Skiploom, Octillery, Remoraid, etc. Then, have them learn Bullet Seed and bring it to the daycare. Put the male Pokemon with bullet seed and female Breloom in there. After you get the egg and it hatches, level up Sheoomish to 45 to learn Spore. Then, evolve it. Finally, go to the PWT with Breloom and talk to the Move Reminder. She'll remaind Breloom Mach Punch.