Let me explain you the cantina dailies as example. Daily quest says, you have to complete 3 cantina battles. The button brings you to the cantina table. It doesn't matter if you do 1-A or 8-G or any stage between. You can select the battle you like. But loosing battles provides you NOTHING. You have to win your battles. So you should pick a difficulty you can 100% win. If you have won a battle with 3star rating (winning stage without loosing a own character) you can afterwards also choose to simulate the battle, so their is no need to do manually the really easy tasks over and over. Always pick a difficulty you can beat. Rancor is the only different thing at the moment. Try it, loose it and don't care. You will not be able to beat rancor below lvl 85, but you will get rewards if you only try.