Yesssss Zekrom that's perfect. Gyarados compliments the first suggestion real well.
When planning a team a big thing to keep in mind, er - two big things - are the roles each pokemon plays, and their typing both offensively and defensively. Especially in singles, competitive matches often involve a lllllot of switching. Be it to force the opposing pokemon out by threatening to KO it, or to 'wall' (which is slang for taking attacks from with little to no danger) the opposing pokemon, and initiate non-directly-offensive strategies, such as spreading status, setting up hazards; etc etc. So you, are best off planning on switching your pokemon, or at least most of them - out to something else on your team providing a number of situations. Namely based on what typings and opponent's roles give each pokemon problems.
Stunfisk, is slow, mildly bulky, and Ground / Electric. It's weak to Ice, Grass, Water, and Ground. But it's immune to electric, and resists Steel, Poison, Rock, and Flying. Gyarados, is an average speed level, not too bulky but not frail, and Water / Flying. It's weak to Electric most prominently, with x4, and rock. It is immune to ground though, and resists water, fighting, steel, and fire.
So if either is your currently active pokemon, due to their typings, they can potentially provide a very good switch in to change the situation. If your opponent tries to attack Gyarados with an electric or Rock attack, Stunfisk can switch in and be immune to or resist the attack. If your opponent tries to attack Stunfisk with either a water or ground attack, you can switch Gyarados in, and either be immune to or resist that attack.
ALSO - the Stunfisk build mentioned above is a Specially Offensive set, meaning it hits things with special attack very hard. The Gyardos set mentioned above is a Physically Offensive set, meaning it hits things with Attack very hard. It's important to gauge and balance those two offensive presences on your team, and often have moves of varying types to hit your opponent with; so as to cover as many situations as possible.
Anywho enough of that; good luck with this mate - and if you ever wanna test it out, hit me up for a 3DS test play match :D
