You need to account for resists, as well as EV the goomy to have equal defenses.
The correct EVs are 184 Defense EVs, Lax Nature. Both Defenses are 167. If it is a resist/weakness, just change its type so that it's neutral. So...
252 SpA Choice Specs Mega Charizard Y Fire Blast vs. 0 HP / 0- SpD Goomy in Sun: 663-780 (287 - 337.6%) -- guaranteed OHKO
Specs Megazard Y measures a 2.87 to a 3.376 on the Goomy scale.
+2 252 SpA Life Orb Solar Power Mega Houndoom Fire Blast vs. 0 HP / 0- SpD Goomy in Sun: 1568-1844 (678.7 - 798.2%) -- guaranteed OHKO
LO MegaDoom after a Nasty Plot in Sun measures a 6.787 to a 7.982 on the Goomy scale.
Let's check out Mega Luke again:
+2 252 Atk Life Orb Adaptability Mega Lucario Close Combat vs. 0 HP / 184+ Def Goomy: 1040-1224 (450.2 - 529.8%) -- guaranteed OHKO
Only a 4.502 to a 5.298. Mega Khan?
+2 252+ Atk Life Orb Parental Bond Mega Kangaskhan Return vs. 0 HP / 184+ Def Goomy: 976-1152 (422.5 - 498.7%) -- guaranteed OHKO
A 4.225 to a 4.987. And just for kicks, let's check this out:
+6 252+ Atk Choice Band Parental Bond Mega Kangaskhan Explosion vs. 0 HP / 184+ Def Goomy: 5527-6504 (2392.6 - 2815.5%) -- guaranteed OHKO
23.926 to a 28.155. DAMN. (Good thing Mega Khan doesn't get Explosion.)
Just for kicks, let's crank up the power all the way. (This Goomy is Fighting-type with 255 Attack. The target Goom is Normal/Dark with 1 HP and 1 Def at level 1. Explosion is fighting-type.)
+6 252+ Atk Choice Band Adaptability Goomy Helping Hand Explosion vs. -6 0 HP / 0- Def Goomy on a critical hit: 12897000-15172944 (117245454.5 - 137935854.5%) -- guaranteed OHKO
1172454.545 to 1379358.545. HOT DAMN.