With hundreds of heroes to unlock, it's pretty obvious that some will be much better than others. And since you can only use a limited amount of heroes in each battle - and XP points are scarce, you should make sure that you're always improving your top heroes only.

This means that you should go for as long as possible without spending XP points - and especially elixirs early on. Go as far as you can by investing as little as possible.

This way, you will unlock more heroes and you will have more to choose from. Select your best then (you can generally understand what your best heroes are in the current setup when unlocking the second stage) and focus on upgrading those.

Usually, the best heroes are those with the highest number of stars. Heroes have a rarity and a total number of stars that can be unlocked: the more, the better. So always try to use the rarest heroes that you have, since those will generally be more useful on the long term.