The power of your Guardian and the effectiveness of their Bonuses depends on the following parameters:
Type
The type of Guardian you should use depends on the action you wish to take. For example, if you're attacking a Pantheon, a "Pantheon Offense" Type Guardian would be the logical choice.
Only Guardians that boost Resource Production, Spying, and City Defense can be activated in the "City" tab of the Guardian interface.
Rarity Grade (I to V)
Guardians are classified according to Rarity. Each Rarity is given a Grade. The higher a Guardian's Rarity Grade, the more powerful they are and the more difficult they are to find.
Rank
In general, the higher the Rank of a Guardian, the more powerful their bonuses will be. However, Guardians at the earlier Levels of a higher Rank are slightly weaker than those at the highest Level of the previous Rank, although they can be improved dramatically.
Level
Ranks are split into Levels. To increase your Guardian's Level, you will need to earn XP. Higher Level Guardians are stronger than lower Level ones, but the maximum Level a Guardian can reach is limited by their Rank.
Rank 1 Guardians can reach Level 15, Rank 2 Guardians can reach Level 20, Rank 3 Guardians can reach Level 25, Rank 4 Guardians can reach Level 30, Rank 5 Guardians can reach Level 30 (35 if enlightened), and Rank 6 Guardians can reach Level 35 (40 if enlightened).
Skill Level (0 to 5 per Skill)
Most Guardians have a set of 4 Skills that affect one of the 4 Unit types (exceptions are the Spying Type Guardian, which specifically improves Spying Units, and Resource Guardians, whose Skills affect Resource Production and Warehouse/Granary capacity).
Enlightenment Level (0 to 5)
There are five stages of Enlightenment, and each one boosts all the Skills of your Guardians. A Guardian with the Rank of 5 or 6 can only reach their highest Level through Enlightenment.
Affinity
All Guardians have an Affinity - Earth, Water, or Fire. Each Affinity performs better against a certain other Affinity. Water beats Fire, Fire beats Earth, and Earth beats Water.
If you send Guardians of different Affinities to a location, a "Dominant Affinity" will be selected to represent your forces.
For battles at your City, you can choose which Affinity you wish to be Dominant. At other locations, your Dominant Affinity will be the single Affinity represented by the most Guardians.
If you do not have a Dominant Affinity, you will not be able to receive the Affinity Bonus to Guardian Skills during battles. If you do not have a Dominant Affinity, but your opponent has Guardians in a battle, and a Dominant Affinity is selected for their army, they will receive the Affinity Bonus regardless of which Affinity is Dominant, as no opposing Affinity will be present.
Sometimes no Dominant Affinity can be selected at battles outside your City. This will happen if no single Affinity is represented by the highest number of Guardians (for example 1 Earth, 1 Water, and 1 Fire, or 0 Earth, 2 Water, and 2 Fire). Make sure to check which Guardians you are sending to a location in order to avoid this scenario.
If your Dominant Affinity beats the enemy's Dominant Affinity, you will receive a 25% bonus to your Guardian's Skills.
Navigating The Guardian Icon
Each of a Guardian's key stats can be seen at a glance by viewing their Icon in your Collection. The stars in the bottom left corner of the icon indicates a Guardian's current Rank. The sphere in the upper right corner represents a Guardian's Affinity. (Gold/green is Earth, orange is Fire, and blue is Water) The number in the bottom right corner of the icon is the Guardian's Enlightenment level. The top left corner will show the Guardian's Level.