Let's start off with an Earthquake, with any luck it'll at least drown out the noise from Screech, an it should have some interesting effects on the arena. Then get in there slow, (Trick Room RULES!) and use Reflect to stop megahorn hurting as much, and finish with Telekinesis to stop it in it's tracks and make it a sitting duck for the next round.

Earthquake ~ Reflect ~ Telekinesis