Adharma is present in every single person, and in the world as a whole too. The issue of fighting adharma is, therefore, a pertinent one. Let me use the example of Vishnu as a pointer towards fighting adharma.
The various avatars of Vishnu have over time, used the same tactics as the adharmic forces to defeat them. Consider:
Parasurama killed the kshatriyas unabashedly in response to mass killings of Brahmins by the kshatriyas. Rama, in his war against Ravana, never deviated from the path of righteousness. He did so, because Ravana himself fought the war justly, following the rules of war as laid down by dharma. Krishna, on the other hand, justified the killings of Karna and Jayadratha (which involved trickery) citing the various actions of Kauravas (disrobing of Draupadi, cheating the Pandavas out of their kingdom, the killing of Abhimanyu) as examples of trickery by the opposition. Adi Shankaracharya, the brilliant philosopher, waged an intellectual war against the widespead prevalance of dogma and intolerance, by defeating representatives from all 72 religions in a debate. He managed to unify all religions under the single umbrella of advaita philosphy, and managed to resolve their differences.
I'm sure there would be many other examples available, but each of the above examples indicates that adharma is always fought using the same tactics of adharma. This is not an abdication of righteousness, but rather the performance of duty without any regard to personal benefit. I'm sure everyone will agree that it is morally incumbent upon us to fight adharma. Adharma must be fought at two levels - the personal level, and the global level, in that order. One cannot set the world on the path of righteousness without the person him(her)self following the same path.
In each case, one must identify the form of adharma, before taking any steps to eliminate it. At the personal level, the actions, as prescribed by Krishna, show us the best way to live. From the Bhagavad Gita (2:47)
which translates as
karmaņy evādhikāras te mā phaleşu kadāchana
mā karma-phala-hetur bhūr mā te sańgo 'stv akarmaņi
You certainly have the right for prescribed activities, but never at anytime in their results. You should never be motivated by the results of the actions, nor should there be any attachment in not doing your prescribed activities.As for the global level, let that be a topic for some other day, some other person.