I used to be able to one hit KO all the pokemon in max raid battles, but now after i spent time progressing the game and defeating the champion i find it that i can barely even get the pokemon to half health. I tried a strategy where i don't take the first chance to dynamax my pokemon and instead wait later in the battle. It worked for a little bit but not too long. I dont play online and all my pokemon are over the level of 75. What are recommended straregies? I can provide you with my pokemon team if necessary.
If you're having to battle with the game-provided AI, a lot of it just comes down to luck -- some of the pokémon are almost dead weight (solrock, wobbuffet, eevee, togepi, clefairy, etc) and their fraility is more of a pain than their usefulness. These kind often don't even help breaking shields!
If you get one of these teammates, sometimes you can manage to get through (depending on how often they get targeted by the raid pokémon), otherwise if you get two often it's worth just quitting out and trying again to save yourself the time.
The AI also isn't very intelligent about type matchups either, and often you'll find it doing stuff like bringing several fire types against a water-type raid. Again, if this happens, sometimes it's just easier to reset.
From your end, the easiest thing you can do is getting your own pokémon to level 100. This will ensure far more survivability and damage output than any kind of slight improvement you get from proper natures, EVs, and so on. All raid pokémon have a maximum level of 60 (the sole exception being that event mewtwo, which is difficult enough even with 4 other humans with level 100 pokémon, let alone trying it alone!).
Ensuring your pokémon has a good type that resists the raid pokémon's moves is more important than making sure you can hit it super effectively. 5-star raids are more about surviving than damage, with strong shields that can even take multiple rounds to get through.
Lastly, try giving your pokémon a (non-consumable) item such as Leftovers or something that boosts the main attack you'll be using.
For specific pokémon, I'd recommend levelling your Zacian/Zamazenta and Eternatus to level 100 and seeing how well that works for you. They have very high base stats due to their legendary status, as well as signature moves that do double damage on dynamax pokémon. They should be able to handle most raid pokémon by themselves, but if you prefer not using legendaries or want to try something else, check out Silvally -- its ability to change type makes it very useful, and its Multi-Attack will change into the appropriately-typed max move if you dynamax it.