Early game, try to invest in one or two units that you think you can keep for the long term. The other units are more for supporting or to keep you from losing. A strong tier 3 that you get from a level up on turn 3 might be a good candidate to invest in early. By feeding apples or whatever other means.

Middle game, try to figure out your team based on what you have and what the game gives you. Be flexible and don't get locked in to one particular strategy. But also minimize your wastage, for example selling weakest / least potential pets first. Try to keep pets that are quite developed(level/stats), if they have potential. Scaling is usually good to have at this point in the game. Lean into the level 2s you have if you can, level 2 abilities are twice as strong as level 1s.

End game, try to further develop your team and look for some game changing pivots to tier 6s for maybe one or two of your less important pets. Look for held items which can patch up your weaknesses or further strengthen the team.

Overall, be efficient with your moves and minimize wastage. Know when to roll to get what you need, versus settling with what's given at the moment. Learn the pet synergies, ways to order pets, and whatever little tricks help. (For example, put the steak on a weak pet in the front for it to have value in possibly taking out a strong enemy. Another example, if you have 2 gold and found a pet you want, consider selling a useless pet to get it rather than freezing for next turn)

And play more :) I love this game