I would say if u have lv 15 diamond just keep using him and speed up the heal. It?s a 25% chance to beat a lv 20 champ (maybe even higher against level 19). You would be expected to beat it in 4 tries which is 20 gold instead of 30. Downside is you have to wait a couple hours for the fight cool down. Everything else you listed other than level 15 metal only has a 10% chance of beating it. That?s pretty bad odds and with some bad luck I can easily see it taking you a whole day. You would be better off using the disposable fire strategy if you don?t have a lv 15 diamond or metal.