Take a few Purple Pikmin (about 10 to 20) and slowly sneak up behind the sleeping Bulborb. Take into consideration the fact that Orange Bulborbs wake up much easier than Hairy and Red Bulborbs. Throw all of your Purple Pikmin onto it quickly, which should stun it before it has time to attack. They will quickly defeat it before you lose any Pikmin. Do not attempt this with the Fiery Bulblax, as the Purple Pikmin will be burnt.