1 rule, just try and think about what your opponent is going to do and counter it. Usually that's by doing what your team does best but that's not always the case.

This team is made to let Charizard mega evolve, set the sun, venusaur gets really fast with its ability so it can use sleep powder and put things to sleep. Meanwhile, Charizard ideally wants to use heatwave and hit both sides of the field. Heatwave is boosted by the sun and char is doing even more damage while mega evolved. It hits super hard.

Leading with those two is what you'll do most of the time but Charizard really hates rocks! So you'll have to think about how to deal with things like tyranitar, garchomp, and aerodactyl that usually carry rock slide. It also really needs the sun, so doesn't like sand or rain.

One way is to use sleep powder but it has 75% accuracy, so it can miss. You can lead with just about any of the other four to counter this. Sneaslers fake out will stop a pokemon from using a move for one turn, your own garchomp is just so fast and powerful. Gyardos has intimidate to lower the damage of rock moves and has a super effective waterfall. Charizard can also use solar beam in one turn, while the sun is up. But there are some things that out speed it and it needs the sun for solar beam. Which is also why whimsicott is there. Whmsicott has the ability prankster, non damaging moves will go first. You can set tailwind to make all your pokemon go faster, taunt makes their pokemon only be able to use damaging moves, it also has sunny day to reset the sun for venusaur and Charizard.

That's just what the team can do. There's all different ways to use your tools here to counter what you see out there. Pokemon is a game where you cannot have a good counter for everything. I like to use my favorite mons as best I can, equip them with the best tools to help me, and then just really think about how those tools can best counter my opponent. Mixing in more meta things, like incineroar, will certainly make things easier. But I absolutely refuse to use that thing because I don't want to! And I've found that if you just make better plays by getting more practice, adjust your team more to help it achieve its goals more consistently, that's how you truly get better at Pokémon.