I first played this game years ago and couldn't beat it. I came back to it today after several years and realized that upgrading stuff isn't the right way to go. The meteor and rain spell cost more mana the more you upgrade it, so it's harder to use. I found that leaving it at level 1 or 3 works best.
I rush upgraded my staff.
As far as warriors, I found it better to not promote them from the menu, instead promoting them in the fight. This is because after each promotion (while fighting), they heal full hp. I kept my warriors at level 1 until I had enough mana to summon 2 of them at level 5. Barbarians are better because they push the enemy back.
Archers are only useful until you are able to purchase level 5 fire mages. After that, just build fire mage in every tower. Unless you're at level 30, I spam built level 5 water mages.
I found it strange that some units had high attack power and high speed (like hunters and fire mages), essentially making those the best units in the game. Multiple attack units can't compare to their counterparts.
This game has stayed in my mind since I couldn't beat it many years ago, probably when it first came out in 2011. I can say that 13 years later, I came back to it and beat it with 3 stars in all missions.