Nasty Plot / Scald / Psyshock / Fire Blast, but tbh I'm considering running Sub > Fire Blast, because:
+2 252+ SpA Life Orb Parental Bond Slowking Scald vs. 252 HP / 0 SpD Ferrothorn: 204-241 (57.9 - 68.4%) -- guaranteed 2HKO
+2 252+ SpA Life Orb Parental Bond Slowking Scald vs. 252 HP / 252+ SpD Ferrothorn: 151-180 (42.8 - 51.1%) -- 50.8% chance to 2HKO after Stealth Rock
+2 252+ SpA Life Orb Parental Bond Slowking Scald vs. 252 HP / 4 SpD Aegislash-Shield: 327-385 (100.9 - 118.8%) -- guaranteed OHKO
You don't really need to run Fire Blast, I can't find anything that Scald / Psyshock don't hit well enough to warrant running Fire Blast.
Also, Mega Gyarados appears to be a counter, resisting its STABs:
+2 252+ SpA Life Orb Parental Bond Slowking Scald vs. 4 HP / 0 SpD Mega Gyarados: 184-219 (55.4 - 65.9%) -- guaranteed 2HKO after Stealth Rock
Isn't Mega Gyarados banned because of Moody? You would have to use something like Sharpedo o.o