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It seems highly unlikely that a purely combat-focused faction can be created from the League. A semi-combat faction, for example, with leadership and attack supported by Chaos or Summoning, is quite possible, but a purely combat-focused faction, with purely combat skills without Chaos and Summoning, perhaps with support from Light and Darkness, is a very questionable option. There are many reasons for this.
The League has higher magical stats. Attack grows decently, but defense is very low. You can only reliably get good attack and luck stats, while the chance of getting defense, leadership, Darkness, and Light is only two percent. In the native castle, it is also difficult to upgrade Light and Darkness spells.
But the problems are not only with upgrading. Almost all racial abilities are also geared towards Chaos; only Elemental Affinity can help a combat unit, but even here there is a problem – this skill is too random. Through Chaos, Elemental Affinity is much easier to use: you can reach any enemy unit, you can choose a spell of the desired element. But with your combat stack, you cannot change the element to the desired one at will, and the enemy does not always have the desired element, or you cannot always reach the enemy with the desired element. And sometimes you will have a choice: attack the most dangerous stacks of the enemy or attack less dangerous ones, but with the desired element.
In addition, when playing with Light or Darkness, high-level magic is almost wasted. No, it works well for some spells, but for many, it is not very effective or not effective at all.
The League's units are also not all combat-oriented: Furies, Lizards, and Dragons. Moreover, the first two do not survive for very long. Assassins or Hunters are not always suitable for this role. Minotaurs are slow and more suitable for a defensive role, and Hydras even more so. Witches can hardly be called a good combat stack.
So, it is unlikely that a purely combat-focused League will defeat another combat-focused faction, unless it upgrades high elemental damage and gets lucky with the right elements. Although, with Darkness, you can also do a lot if the opponent does not have protection against this magic. But still, with good combat magic support, this faction has a much higher chance of winning.