I've seen some members talking wrong things about Mischievous Heart, so I retested it and debunked what they've said:
Testing with a lvl 44 Erufuun: Energy Ball / Stun Spore / Toxic / Leech Seed. Battle against a lvl 72 Garchomp.
Garchomp used Dragon Rush!
Garchomp's attack missed!
Erufuun used Energy Ball!
This is the confirmation it was faster. Then, next turn:
Erufuun used Leech Seed!
Foe's Garchomp was seeded!
Garchomp used Crunch!
Erufuun fainted!
This is the confirmation that Leech Seed works with Mischievous Heart. I then sacrificed a member of my team to use revive on Erufuun.
Erufuun used Toxic!
Foe's Garchomp was badly poisoned!
Garchomp used Crunch!
And this confirms Toxic works with Mischievous Heart, too.
Then, against a level 75 Milotic.
Milotic used Blizzard!
Erufuun fainted!
This is the turn I used Energy Ball. Milotic is faster, then. I, again, revived Erufuun through a sacrifice.
Erufuun used Stun Spore!
Foe's Milotic was paralyzed!
Milotic is fully paralyzed! It can't move!
And this proves Stun Spore is affected, too, by the ability.
I just need to find the Taunt and Substitute TMs then I'll test them. I've also teste Double Team, Flash and Cotton Spore earlier, and the three of them were affected.