While Mandibuzz definitely lost a lot of its viability during the last months, due to the absence of Aegislash and the prominence of wallbreaking MEvos that shit all over it (Mega Charizard Y, Mega Gardevoir, Mega Heracross), it's far from a bad Pokemon. I would drop it to B, but not any lower, as it's still a decent Defog user, checks many important threats such as Landorus, Mega Scizor, Excadrill, Latios, Latias (that lack Thunderbolt), Landorus-T, Talonflame, Dragonite, Garchomp, Gengar, Mega Alakazam, Breloom, and regular Gyarados, and has many useful support options, such as Taunt, Whirlwind, and Knock Off. It can even forgo Defog to run a set of Taunt / Toxic / Roost / (Foul Play / Knock Off), which is a decent stallbreaker that checks a unique array of Pokemon. Yeah it needs anti-SR support, but that's not really hard to provide and a must for most teams. It sucks being so reliant on SR being off the field though, because Mandi fails to deal with a lot of threats if SR is on the field, which makes it pretty match up reliant. If it can beat the opposing SR setter (Ferro, Lando-T, Garchomp, Tyranitar), that's nice, otherwise things are going to be a bit difficult for it.
Terrakion is a monster, all Lando-T sets are easy to wear down, and everything else can be beaten with very little previous damage. SD + dual STABs + filler is Terrakion's best set atm, and if you go with Sub, Slowbro has to bank on the 30% burn chance of Scald to not lose, which is quite a shitty position to be in. Terrakion is a very consistent wallbreaker from my experience, that helps soften up teams early game for my cleaners, or just toys with balanced and some stall teams. It's one of those Pokemon that is useful no matter the matchup you face.
Breloom should drop because all the MEvos that got popular recently fuck it up (Mega Zard Y, Mega Pinsir, Mega Gardevoir, Mega Heracross, Mega Medicham), including Pokemon that can anti-lead its Sash set, namely Fake Out Mega Medicham and Jolly Mega Heracross. SD + LO is really fun though, and wears down bulky Grass-types like a charm, so if people start catching up on this set (which is actually hard to wall and punishes switches) it may be worthy of A- again.