I did some calcs with Tyrantrum, and while it's probably not the greatest threat considering its weakness on the special side and typing, it can get work done.
+1 252+ Atk Tyrantrum Head Smash vs. 252 HP / 252+ Def Mega Sableye: 237-279 (77.9 - 91.7%) -- guaranteed 2HKO
This shows that tyrantrum will be able to put a massive dent in unresisted foes. Normal Sableye will be OHKO'd before it could try to use prankster Will-O-Wisp, and Mega Sableye will be too slow to not get KOd the next turn
+1 252+ Atk Tyrantrum Head Smash vs. 252 HP / 88+ Def Ferrothorn: 129-152 (36.6 - 43.1%) -- 98.2% chance to 3HKO after Leftovers recovery
Ferro will still wall it, but that's what your team is for
+1 252+ Atk Tyrantrum Head Smash vs. 252 HP / 252+ Def Slowbro: 259-306 (65.7 - 77.6%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ Atk Tyrantrum Head Smash vs. 252 HP / 252+ Def Mega Slowbro: 180-213 (45.6 - 54%) -- 46.9% chance to 2HKO
Head Smash will dent the Mega Bro, but is still best off with team support.
252+ Atk Tyrantrum Head Smash vs. 252 HP / 252+ Def Unaware Clefable: 225-265 (57.1 - 67.2%) -- guaranteed 2HKO after Leftovers recovery
Even unaware Clefable cannot switch in on this beast. Moonblast will OHKO Tyrantrum, but Clefable cannot switch in on it
+1 252+ Atk Tyrantrum Head Smash vs. 4 HP / 252+ Def Eviolite Chansey: 508-598 (79.1 - 93.1%) -- guaranteed 2HKO
Another non counter
Ultimately I'll see ferro usage go up to counter Tyrantrum, as well as really anything with good defense that resists the Smash
With these calcs in mind, an adamant +1 Tyrantrum should reach a speed stat of 361, speed tying max speed base 115 Pokemon. I haven't checked what jolly can do yet, but someone can do the calcs to ensure it gets the 2HKOs it wants while being able to outspeed base 130 Pokemon at +1 max speed
