While I myself am not very good at competitive either, I do know a few things and can TRY and help you, though there are probably better people than me to help you. I'd recommend checking out Smogon's entries on each of the Pokemon on your team as they usually have good advice on specific sets and can really give you some good team building strategies. Here's my take on your team:
Mandibuzz - Your Mandibuzz is clearly meant to stall as well as dispel all hazards, however in my opinion only one attacking move makes it far too predictable. Steel Wing can give good coverage against fairy types, or Brave Bird can be used as a good flying type STAB move, which I feel Mandibuzz would benefit from as it has more than enough HP to take the recoil. As for moves to replace, I feel like having a spinner AND defogger is a bit overkill, as I personally have never found entry hazards to be that much of a problem.
Ninjask - Listen, there's no way Ninjask is going to survive the OU competitive. Having a focus sash does help it as it is guranteed a move, but Ninjask does not have high enough attack to put a dent in many teams. Also, it has Baton Pass which is banned in most competitive formats, and Ninjask doesn't have the bulk to effectively use baton pass anyways. Volcarona is in my opinion a good bug type replacement, as it's special attack and bulk do a good job of filling Ninjask's place, and because it's fire type it has good coverage. I'm not sure what you would want to put on Volcarona, as it can both be a somewhat decent special wall with Light Screen, and can do all out damage with good special STAB moves. I would personally go for a mix of the two, but that's just me.
Garchomp - Garchomp is a really good sweeper with enough bulk to set up and a monsterous attack stat. The combination of rough skin and rocky helmet is pretty good but leaves it vulnerable to special moves, which will be less of a problem if Volcarona sets up with light screen. I would replace Dragon Tail with Iron Head for coverage though, since fairies are a constant threat in OU.
Greninja - A pretty good set for Greninja, however I would replace extrasensory with Scald for a burn chance and 100% accuracy. I know it sacrifices a bit of coverage, but extrasensory without stab is really bad.
Weavile - Honestly just a really good set for Weavile, though having two dark types does leave your team vulnerable to fighting and fairy types.
Blastoise - Once again, why two of the same type? Besides that, I would add Earthquake to have coverage against the ever present electric types. I know this is a mostly special set, but 103 attack isn't bad.
Something I would like to add is that you should replace either Greninja or Blastoise for more coverage, and also the fact that you have three dark types, so you should either replace Greninja to kill two birds with one stone, or hard counter it more effectively.
