I have been using this set lately :
Mandibuzz@Leftovers
252 sp. def/252 HP/4 def
Set :
Toxic
Taunt
Roost
Foul Play
I like to run this set, because with max special defense, you wall pretty much almost everything in the tier, bar those super effective moves. Toxic is a very effective move to run on this set, as it allows you to cripple those walls that think they can switch into Mandibuzz and you can cut the life short of those sweepers like Ludicolo and Carracosta, who think they can take it down. You can then go for the taunt to prevent them from even setting up that Shell Smash or that Rain Dance on the switch, allowing you to be able to cripple them before they switch out. But you do have to look out for the ice beam or any super effective move against Mandi or any move that hits on the physical side on this set,because that will shorten Mandibuzz's life line. Jynx is a huge problem for this set,if she switches in to revenge kill it. Foul Play can surprise those Golurk that think they can switch in on Mandi, expecting a brave bird and wham ! Golurk is OHKO'd by Foul Play due to its high attack, and then you can say goodbye to the stone Golem. More problems for this set would be faster taunters like Misdreavus(I think) or pokemon with prankster like Murkrow or even Liepard. Also, pokemon like Golem and Regirock completely wall this set, even though it has Toxic, they can hit with a super effective rock slide or even a rock blast, denting Mandibuzz for the remainder of the match. This is where Roost comes in. Roost allows to heal that HP that you lost from those attacks and if they attack again, it won't be super effective due to the loss of the flying typing for the turn. Roost also helps to stall even further with this set, so from my experience, Mandibuzz is an amazing stallmon so far, and I do expect it to be more used. But one more problem for Mandibuzz would be SR, because since she is part flying,she will be taking damage from Stealth Rocks every turn, therefore shortening her life line, but you can use roost to heal off that damage, meaning that you can switch in more.
Status is a bigger problem for Mandibuzz though, as I had a battle where a guy paralyzed my Mandibuzz with a T-Wave coming from a Rotom-F(don't know why and I switched in expecting him to switch out), and it did cripple Mandi, but she was still able to perform her role, just not as effectively. Toxic is a huge problem for Mandi, because that will wear her down to the point where she can not do her job for the remainder of the match.
I have also found that you can run Whirlwind over Toxic, but that only helps in a phazing team, which I was not using Mandi in a phazing team, but more of a balanced team.
(I am just starting posting,so this might be wrong)