The Bug is a little more difficult but the same Idea as the Gulper. Head over to Tirna Seith in Ardenweald. The outside world not the instance just in case you are confused. I recommend taking a few people once again so you can farm the deranged guardians in the misty area Tirna Seith.
This will mount the spriggens on top of them. Once the spriggens die they have a chance to spawn a mob called Tizo. You will see them Yell out hey Tizo their killing us come help us or something. It can take a very long time. Some reported 7 hours of farming. We took about 30 min with 4 people.
Once he spawns, he will be on a mob called Chomper in the area and you have to kill Chomper to kill him like the guardians. You will need another large group because he is quite a pain to kill. Once you kill it though everyone that kills it gets that 100 percent mount drop.