How much damage will this do?

Question

Say I have a Mega Mewtwo Y battling a level 1 Happiny Level 100Mewtwo Item:Mewtwonite Y
Nature:Modest Ability:Insomnia
EVs:252 Sp.atk 252 Speed 4 HP
-Calm Mind
-Psystrike
I use calm mind 6 times then use Psystrike
Level 1 Happiny Item:None
Nature:Hasty Ability:Friend Guard
EVs:None
-Pound
-Charm

Answer 1

+6 252+ SpA Mega Mewtwo Y Psystrike vs. 0 HP / 0- Def Happiny: 57300-67413 (440769.2 - 518561.5%) -- guaranteed OHKO
Source: http://pokemonshowdown.com/damagecalc/