I think the Sigilyph moveset might need to be changed. Psyshock is pretty useless for anything but Haze Cryogonal. Munchlax isn't really going to do anything to a team so I wouldn't worry about it. Calm Mind, Air Slash, Roost, and Hidden Power Ground / Heat Wave is much better suited for coverage and it certainly needs roost if it wants to get all the boosts it can and set up on things like Slowking.
Mandibuzz @ Leftovers
Trait: Big Pecks
EVs: 248 HP / 252 SDef / 8 Def
- Brave Bird
What it does:
It breaks stall with taunt and walls tons of special attackers all day with it's huge special defense and hp. Pokemon like Accelgor, Liligant and Sceptile have nothing on it even with Hidden Power Rock. There are many Pokemon such as Aggron or Aerodactyl that switch in with no fear at all of being touched. This is when Mandibuzz should predict and use Whirlwind to keep wearing the opposing Pokemon down till it can no longer switch in. Mandibuzz should be used on a stall or a balanced team as offensive teams won't be able to handel Mandibuzz's switch-ins.
Steelix is a great teammate for its incredible defense and its immunity to Electric and resistance to Rock allowing it to take things like Aggron's Head Smash. It has Roar, and a hazing core is always good for not letting sweepers setting up. And of course it has Stealth Rock which is a must for Mandibuzz to be able to deal damage to physical powerhouses on the switch in and Whirlwind them away so that they must take damage again to switch in. For this reason, a spiker is also very necessary as a teammate of Mandibuzz. It's weakness to Rock also requires a Rapid Spinner if it wants to be able to stop all it can stop.
What counters it:
Aggron and Magneton resist Mandibuzz's only attack x4 and aren't affected by Toxic. Head Smash will wear down Mandibuzz's partners aside from Steelix, and Magneton will trap Steelix. Physical attackers that resist Brave Bird like Kabutops and Aerodactyl will always do the trick. Having a good Rapid Spinner will always stop Mandibuzz from doing what it does best.