Don't want to exhaust your dragons by endless battles against Helios, Dark Angel and Diamond? Buy lots of Fire Dragons (100$, cheaper than healing & you won't even have to feed them) and battle with those instead. It doesn't matter if they lose -- just sell them! The odds are the same for all non diamonds anyway, so why not play it cheap?
And yes, it works, just beat Diamond lvl 20 with a lvl 2 Fire