I have one question. It seems like whenever I beat the elite four it's..

It helps to have your Pok�mon above Level 60 before battling the Elite 4. Your team should have strong Pok�mon like Gengar, Alakazam, Charizard, Snorlax, etc. Use the Vs. Seeker to rebattle strong trainers. Otherwise, train in Victory Road.

I think your post is flawed. Zapdos, Articuno, Moltres and Mewtwo are four Pokemon, tha tmakes for two spots left.

Anyways, I highly suggest training in Victory Road with EXP. Share. That will level you quickly. I suggest Leveling yourself past Level 63. Your Rival's Starter is at 63, I highly advise hitting 70 for best results.

Try getting your Pokemon to around level 70. Level 50 won't get you far at all.

You don't need to focus on getting certain Pokemon. It sounds like you need to worry about your strategy. For instance: have as many types represented in your party as possible. Using dual types can help to bring in more variety. Next, you should make sure you have many types of attacks so you can handle different opponents. Also include moves that heal, cause status problems or up your evasion. Disabling your opponent with a stat problem or boosting your Pokemons' defences can buy you time to attack more. Also think about your opponents' abilities. If you consider its' attacks you will know which Pokemon to use that will be least effective. Next, you should know your Pokemon. Make decisions based on what you know your team can do. Here's an example: My Team consists of Charizard, Nidorina, Vaporeon, Raichu, Blissey and Parasect. Between all of them I have Fire, Flying, Poison,Normal, Electricity, Water, Grass and Bug types present. Now they all have one attack that matches one or both of their types plus one or more of another type. For instance, Blissey has Ice Beam and Psychic, Charizard has Earthquake and Flamethrower, Nidorina has Crunch, Water Pulse and Super Power. Raichu has Focus Punch etc. Also I had them learn moves that either enhance their abilities or caused stat problems. One example of strategy is when I faced Agatha (Ghost Trainer E4) I used my Chansey,now Blissey, to face her knowing her ghost Pokemon couldn't use Ghost attacks against a normal type. Also I used Minimize a few times to protect against anything that could hit. Then Softboiled if I took damage. Then I had ample time to attack with Psychic and Ice Beam. I beat her using ONLY Chansey. So the point is to plan your attack strategy effectively, Disable your opponents with stat problems and have a team that is diverse in both Pokemon types and move types. I hope this helps.