Bronze refresh isn't much, but it can make quite a difference sometimes. Firstly, you will save up on some tCrystals. Secondly, you can get an achievement for using them a certain amount of times. And, thirdly, there is always a chance that even a Bronze refresh will give you some higher level heroes.

Elite missions drain your power way quicker, so think long and hard whether they are worth it. Sometimes they are just a waste of time and will stop you from carrying on.

Don't create a homogenous army. Every unit type has its strong and weak points, so you need a well-balanced group to take out your enemies. There's no such thing as a single most powerful unit in the game.

That being said, you should always place emphasis on your best warriors and spend your upgrades on them. They are the ones you will keep long-term, so it is only logical to make them a strong basis for the rest of your army.

There's no real reason to replay older missions to get resources. The chances are, your army has already surpassed the level of items these missions offer. You will only lose power and have to wait for your warriors to recover.

If you choose to farm though, see that it is at your level or at least close to it. Also, make sure there is a possibility of an item being dropped.