Energy is the Resource that must be spent to fight Warlords. Each battle requires 10 Energy.
There are 20 Levels of Warlords. In most cases you will maximize your rewards (Guardian XP, Summoning Bonds, and Energy) if you defeat a Warlord in a single attempt.
Each Warlord has a certain amount of HP. In turn, each of your Guardians has their own Attack Power statistic. If the Guardian's Attack Power is greater than the Warlord's HP, the Guardian will defeat the Warlord.
When fighting Warlords, only the Level of your Guardian is important. A Level 15 Guardian of Grade III, Rank 6, will be just as effective as a Level 15 Guardian of Grade I, Rank I.
When deploying your Units on Offensive or Defensive missions, you can send Guardians whose type fits the mission. This will boost the army's combat statistics.
The Bonuses given by a Guardian are calculated on top of other Unit-enhancing bonuses, meaning that an upgraded Guardian can provide a huge boost to your army.
Guardians at your City
You can station up to 3 Guardians at your City to strengthen City Defense, or to improve Resource Production.
There may be many Guardians in any given battle, but your army can only be represented by one Affinity in each. When 1 or more Guardians are activated at the City, you can choose a "Dominant Affinity". If you do not choose a Dominant Affinity, deactivate the Guardian with the Dominant Affinity, or this Guardian is stunned, the army defending your City will remain without any Affinity.
If your City is successfully attacked, the Guardians you had activated at the time will be left stunned.