Have a variety of Pokemon types. Seriously. I'm not even kidding.
You are not, and I repeat, am not a gym leader. You are a normal person who goes about their daily lives battling gym leaders.
So I can pretty much guarantee you that if you have a team of Dragon-Type Pokemon and go and challenge that Ice-Type gym, you will not win.
The best types of teams are the ones that cover each others' weaknesses and have multiple uses.
Heres an example: (BTW this really happened)
Trainer A and Trainer B (Me and my friend) were doing a team battle against Trainer C and Trainer D (My other friend and his friend). For the past week, A and B were planning their strategy. They thought, C will most likely use that Lv. 100 Darkrai and his Lv. 100 Lucario, so lets find a Pokemon that can go against both of those. They settled on the idea of A using her specially trained Mega Mewtwo Y, with the special ability Insomnia and its EVs set to the point that it would always move first. They also chose two Pokemon that could go up against D's (Hacked BTW) Volcanion and 3 other Pokemon to cover any weaknesses.
Trainer A foolishly told her brother about the plan (But never mentioned the part that Physic-Type Mega Mewtwo Y has the type advantage over Fighting-Type Lucario) who then told C.
And when you think that everything is at rock bottom, that Mega Mewtwo Y that you had out for the Darkrai that never came annihilates C's Mega Lucario! Wow!
So in the end, A and B won on account of having Pokemon that made up for each others' weaknesses and had more uses than one.
Seriously, this was a true story. I have the video to prove it. (I'm not recording it right now though BC it's late.)
And not we all know how important variety is!!!
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Pokemon Gaming Tips
RandomSelf explanatory. If you are failing at playing that new Pokemon game that you got for Christmas, this book is for you. Tips and hints by yours truly about how to clear the game with ease.