outside of playing the market for which you need to have a significant amount of adena to start with, the only reasonable way to farm some adena is by farming mark of battle by running 105 toons on olympiad, with marks you can purchase skill books that sell for some adena
only issue is that if you are really good and have enough toons to run full oly per month you are looking at getting lets say 60-ish billion, which at current nccoin conversion prices is 30USD, is it really worth spending 16h (i think its 16h of olly per month but correct me if im wrong) of hard work per month to make 30usd?
better buy ncoin and sell event boxes.