You can use 3 items in Event Stages.
1. Heal: Heals deployed Rangers.
2. Shield: Makes deployed Rangers invincible for a set amount of time.
3. Speed Up: Raises deployed Rangers' Move Speed and ATK Speed for a set amount of time.
4. Bombard: Damages enemies and Towers within a certain range.
Earn items by destroying Towers.
Items will help you out in battle so make sure to use them!