Choose your starting heroes according to the enemies you're facing. If you can't confidently predict how the enemy will attack, it can be a good idea to select neutral heroes - the ones that are neither stronger nor weaker against that specific type of enemy.

When you get three stars in an area, do the cleaning as soon as you can, because you'll get a number of items.

You can summon a monster for free every two and a half days, so make sure you use it. Keep track.

You can't replay mastered stages, but you can make sure you have three stars on every level.

Watch a couple of ads to get some free things.

Finally, when you get an artifact, you should always give it to your best character or, at least, to the one you use most often.