In regards to Mega Medicham I don't think he's viable at all, swap in a ghost and he completely pancakes himself with HJK. Maybe a mixed set with Shadow Ball might work, but every then you need great prediction and is base 80 SAtk going to 2HKO most ghosts anyway? Maybe, but probably not.
252 Atk Pure Power Mega Medicham Fire Punch vs. 252 HP / 0 Def Aegislash-Shield: 192-228 (59.2 - 70.3%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Pure Power Mega Medicham Fire Punch vs. 4 HP / 0 Def Gengar: 206-243 (78.6 - 92.7%) -- guaranteed 2HKO
252 Atk Pure Power Mega Medicham Psycho Cut vs. 4 HP / 0 Def Gengar: 576-680 (219.8 - 259.5%) -- guaranteed OHKO
If they switch-in on the Bulk up...
+1 252 Atk Pure Power Mega Medicham Fire Punch vs. 252 HP / 252 Def Trevenant: 384-452 (102.6 - 120.8%) -- guaranteed OHKO
+1 252 Atk Pure Power Mega Medicham Fire Punch vs. 252 HP / 0 Def Aegislash-Shield: 288-340 (88.8 - 104.9%) -- 31.3% chance to OHKO
252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. +1 0 HP / 0 Def Mega Medicham: 159-190 (60.9 - 72.7%) -- guaranteed 2HKO
Who needs Shadow Ball? If the Ghost isn't dead, cripple it on the switch-in with one of the elemental-punches. If the opponent's ghost is obviously slower than you, use Bulk Up and then KO it as it switches in.
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You're looking at a `mon with 100 base speed and 598 attack. After a
bulk up, Mega-Medicham does
more damage on non-STAB moves than Mega-Lucario
Swords Dances.
252 Atk Pure Power Mega Medicham Thunder Punch vs. 252 HP / 252+ Def Slowbro: 184-218 (46.7 - 55.3%) -- 99.6% chance to 2HKO after Stealth Rock and Leftovers recovery
252 Atk Mega Lucario Crunch vs. 252 HP / 252+ Def Slowbro: 128-152 (32.4 - 38.5%) -- 98.6% chance to 3HKO after Stealth Rock and Leftovers recovery
Even at a 5BP disadvantage, Mega Medicham hits Slowbro harder at +0... and even at +1 Medicham vs +2 Lucario.
+1 252 Atk Pure Power Mega Medicham Thunder Punch vs. 252 HP / 252+ Def Slowbro: 276-326 (70 - 82.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+2 252 Atk Mega Lucario Crunch vs. 252 HP / 252+ Def Slowbro: 256-302 (64.9 - 76.6%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Obviously, in the long term, Swords Dance will deal more damage than Bulk Up. But the fact of the matter is, +1, and +2 Mega-Medicham is roughly comparable to +2, and +4 Mega-Lucario in terms of damage (slightly more powerful at one-turn of setup, slightly weaker at two-turns of setup). At +0, Mega-Medicham deals significantly more damage.