1)The most easiest way is to let yourself be ring outed by hercule in the 
mission "Let Hercule Win!" these mission is in the "Majin Buu Saga" if you 
clear these mission you will earn 60,000 zeni and if you equip WE GOTTA POWER 
skill at Level 5 skill you will earn 120,000 zeni(make sure you make the difficulty 
level is "Z") Note:You should by the Z difficulty in the Warriors Room.

2)You should have these skills do buy these accomplish No.1 Fighter's 
Body=Increases maximum health,Veteran Warrior=Increases Power and Defense and do 
these combo its very useful,Square,Square,Square then use Left O.