If you want your guardians to focus on a single enemy to annihilate him first, you can simply tap him. During that one attack, they will all go fight him, which can be useful to take down tougher units.

Use this type of attack to get rid of dealer-attack type of enemies who are usually all the way to the back.

You can also order one specific guardian to fight a monster by dragging your finger towards it.

On the other hand, tapping a guardian will restore his health points, so use it whenever they're getting low.

Try to destroy the Tower of Summons before other monsters appear because it's their spawning point.

You should also get rid of the Tower of Heal as soon as possible, otherwise you won't be able to defeat the monsters efficiently.

Learn the guardian types. Tankers are good at absorbing damage, Ranged Dealers shoot at enemies from the back, while the Aid Guardians offer support during battle.