There are a few types of modifiers that affect the outcome of a battle directly:
Research them in the War Temple, these boosts will increase all stats of the specific troop (damage, defense, siege, elemental force, and loot capacity).
Those are essentially Boost items that you can activate and which work for several hours.After that, the effect will expire and your armies will return to their base values (potentially boosted by other factors, like the usage of a specific Guardian).
Your City Guardian can add a specific bonus directly to your troops. They are very effective to improve troops' attributes like defense. These boosts apply to all troops in the city, or all troops that take part in an attack containing that particular Guardian.
For example, these are bonuses provided by a level 19 Entmoor, evolved to 2 stars:
As you can see, the last position is greyed out. This is because it will only become available in the final stage of Entmoor's evolution.