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The only way I can see a Nidoking mega working is to give it a new ability, make it a full physical attacker, and give it a few more physical moves. Then the opponent won't know if you're running special sheer-orb Nidoking or physical Mega Nidoking.
>Hp: 81
>Att: 135 (+33)
>Def: 110 (+33)
>SpA: 85
>SpD: 80 (+5)
>Spe: 114 (+29)
>Ability: Piercing Strike
>All stabbing and piercing moves become 100% accurate and bypass opponent's defensive moves.
Defensive moves consist of: Protect, Detect, Spiky Shield, Crafty Shield, King's Shield, Mat Block, Quick Guard, Wide Guard, Substitute, Endure, Reflect, Light Screen, and Safeguard (as in, Poison Jab can still poison someone protected by Safeguard). This ability is basically an upgraded Infiltrator that only works for the few moves below.
The following are piercing/stabbing moves (basing these off of in-game descriptions). Those with an asterisk next to them are new moves that would be added to Nidoking's move pool. Two asterisks means Nidoking can already know that move. The others would be useful for other Pokemon that could get this ability (Mega Beedrill/Mega Excadrill?).
>Drill Peck*
>Drill Run**
>Fell Stinger
>Fury Attack**
>Horn Attack**
>Horn Leech*
>Ice Shard (this one's a maybe)*
>Megahorn**
>Peck**
>Pin Missile
>Pluck*
>Poison Jab**
>Poison Sting**
>Spike Cannon
>Twineedle
For obvious reasons, Horn Drill would be excluded. The mega design would feature much greater prominence on Nidoking's horn, but would be sleeker (because of the increased speed) and have a bit of an armored appearance (because of increased defense).
Ok. There's my attempt at making Nidoking a physical attacker with a unique twist to differentiate him from the millions of other physical attackers. He'd pretty much be a stall breaker, bypassing all the wish+protect or substitute abusers. But he still dies very easily to any decent special move, especially water and psychic.