My Pokemon is so no good at all: type disadvantage and ability isn't working. :/
Let's start with Megahorn! For the second action, if the enemy is still standing on the field with no Substitute or copy of himself, use Drill Run. If the opponent is detect/protecting himself, then use Focus Energy instead. If there is a Substitute or copy of the enemy on the field, always use Earthquake to take them all down.
Megahorn/Focus Energy/Earthquake ~ Drill Run/Focus Energy/Earthquake