I'm going to add my opinion however I'd like to say that HellDoom gave an excellent answer, with Infiltrator Crobat / Thundurus-T.
Alright so, first off both Gliscor and Mandibuzz have great physical bulk. Since most are physically bulky, using something like Weavile or Mamo wouldn't neccesarily be bad, but Gliscor/Mandibuzz are still able to take their attacks
252+ Atk Weavile Ice Punch vs. 252 HP / 252+ Def Mandibuzz: 180-212 (42.4 - 50%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Weavile Ice Punch vs. 244 HP / 248+ Def Gliscor: 316-376 (89.7 - 106.8%) -- 43.8% chance to OHKO
252+ Atk Mamoswine Icicle Crash vs. 244 HP / 248+ Def Gliscor: 376-448 (106.8 - 127.2%) -- guaranteed OHK
252+ Atk Mamoswine Icicle Crash vs. 252 HP / 252+ Def Mandibuzz: 212-252 (50 - 59.4%) -- 77.3% chance to 2HKO after Leftovers recovery
It's more of a problem against Mandibuzz, due to it's ability to then take an Ice Shard from either of them and roost off the damage, stalling them out if neccesary. Mamoswine also doesn't like facing Gliscor behind a Substitute, unless running Icicle Spear.
Anyway bla bla bla you're looking for offensive counters right? Don't expect to have a Pokemon which can OHKO both Mandibuzz and Gliscor. Mandibuzz's good special bulk prevents such a thing from happening. However Pokemon like Rotom-W can deal hefty damage with Hydro Pump and Volt Switch / Thunderbolt
56+ SpA Rotom-W Hydro Pump vs. 244 HP / 0 SpD Gliscor: 366-432 (103.9 - 122.7%) -- guaranteed OHKO
56+ SpA Rotom-W Thunderbolt vs. 252 HP / 0 SpD Mandibuzz: 246-290 (58 - 68.3%) -- guaranteed 2HKO after Leftovers recovery
56+ SpA Rotom-W Volt Switch vs. 252 HP / 0 SpD Mandibuzz: 192-228 (45.2 - 53.7%) -- 2% chance to 2HKO after Leftovers recovery
Mandibuzz can still roost off Thunderbolt damage though due to higher speed, unless you're running a fully offensive Rotom-W (eg. Scarf) In which it would 2HKO Mandibuzz and OHKO Gliscor no problems.
Also like HellDoom said Thundurus/Thundurus-T are also good offensive counters;
252 SpA Thundurus Thunderbolt vs. 252 HP / 0 SpD Mandibuzz: 300-354 (70.7 - 83.4%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Thundurus-T Thunderbolt vs. 252 HP / 0 SpD Mandibuzz: 366-432 (86.3 - 101.8%) -- 12.5% chance to OHKO
252 SpA Thundurus Hidden Power Ice vs. 244 HP / 0 SpD Gliscor: 324-384 (92 - 109%) -- 56.3% chance to OHKO
252+ SpA Thundurus-T Hidden Power Ice vs. 244 HP / 0 SpD Gliscor: 396-468 (112.5 - 132.9%) -- guaranteed OHKO
If you want to look for more defensive counters to those two, taunt users are the way to go. Skarmory variants with enoguh speed to outspeed and taunt, or against tauntless Gliscors and stuff wall it to no end. Sableye can take on both, Will-o-wisping them and taunting them, watching them die slowly while recover stalling.