I thought about it(Storm Drain), but I decided to deviate from mechanics with this one due to common sense purposes. Storm Drain is based around water being pulled towards it. Numel is standing right in front of Megawhal to protect it, and things which are attracted don't go up and down an obstacle, they instead hit the obstacle head on. THis particular obstacle isn't fast either.

HP: 5
Atk: ****(+)
Def: **
SpA: *(-)
SpD: **
Spe: 45
Energy: 68%

HP: 68
Atk: *(-)
Def: **
SpA: ***(+)
SpD: **
Spe: 65
Energy: 68%

HP: 34
Atk: Rank 3 (+)
Def: Rank 2
SpA: Rank 2 (-)
SpD: Rank 3
Spe: 85
Energy: 82%
20% PAR
-1 Atk

HP: 70
Atk: Rank 4 (+)
Def: Rank 3
SpA: Rank 1 (-)
SpD: Rank 2
Spe: 31
Substitute(20)
Energy: 59%
-1 Atk

HP: 30
Atk: Rank 2
Def: Rank 2
SpA: Rank 4 (+)
SpD: Rank 2
Spe: 29 (-)
Energy:72%
-1 Atk
Epsilon prepares a
Wild Charge, and begins to run forward. This is intercepted by a
Sucker Punch from Megawhal! THis is too much for Epsilon to bear, and it falls down in a dead faint. Omicron attempts revenge, but realising Tormeldon is protecting Megawhal, chooses to
ENergy Ball it. Firmoise takes the chance to use
Leech Seed on Omicron, and Tormeldon uses
Flamethrower, which leads to a
burn!
The next turn, Omicron uses
ENergy Ball on Megawhal, leaving it barely conscious. Firmoise, safe, uses
Toxic on Omicron. THis is followed by Tormeldon's
AncientPower, and Megawhal's
Rock Slide, leaving it just hanging on.

HP: KO
Energy: KO

HP: 10
Atk: *(-)
Def: **
SpA: ***(+)
SpD: **
Spe: 65
Energy: 56%
Leech Seed(4 actions)
Toxic(level 2)
1st degree burn

HP: 14
Atk: Rank 3 (+)
Def: Rank 2
SpA: Rank 2 (-)
SpD: Rank 3
Spe: 85
Energy: 71%
15% PAR
-1 Atk

HP: 73
Atk: Rank 4 (+)
Def: Rank 3
SpA: Rank 1 (-)
SpD: Rank 2
Spe: 31
Substitute(20)
Energy: 53%
-1 Atk

HP: 20
Atk: Rank 2
Def: Rank 2
SpA: Rank 4 (+)
SpD: Rank 2
Spe: 29 (-)
Energy:68%
-1 Atk