Epic War 4 is a lot about grinding, at least on later level or higher difficulty.
On normal without grinding you will need to quickly buy 3 heroes, then get more troops.
The best troops in my opinion are :
-succubus, they throw up any enemies at a distance, very good when you're facing strong and numerous waves.Upgrade succubus to get more of them.
-the girls that throw blue fireball (don't remember their names), their health is very weak, but their fireball are great as they can push enemies back, and once upgraded they are very powerfull.
-valkyries, those are very resistant, they are quick and deal lot of damage, again upgrade them to get more of them. Valkyrie can only be unlocked by opening the upgrade on the right that need first you to get a hero on level 3.
-tiger warriors, they seem to fall quicker than a valkyrie, but they pack an insane amount of punch when they hit (one hit and usually a wave of enemy is dead), and they are very fast. Same as valkyries, can only be obtained from the upgrade branch on the right.
-epic canon, same as succubus, their area damage and "throw enemy up" makes this a great asset to deal with strong and numerous waves.
Titans are fun units, but you don't need any of them so use the money on troops upgrades, usually if you have the best troops and have upgraded them enough time, you'll have destroyed the enemy fort before they reach the fortess.
The Shield spell is the best spell of the whole game, making your soldier invulnerable for a time.
Be sure to get it and the altar upgrade (that improves your mana stock and mana speed).
the Shield and the Altar upgraded to the max are an extremely easy way to win every stages.
Just wait in your castle until you are full mana, then charge, use the Shield spell when you are near an enemy Titan/big bang bomb/dark goblin/etc... each time the shield counter reach 3, launch the spell again.
In mission that features an invulnerable enemy (dark goblin, celestial angel by example), focus on the fortress and the Shield spell, do not care about what happens around the invulnerable boss, when the fortress goes down, the boss is dead.
Grinding is needed to get the troops to be high level enough for later difficulty levels.