Don’t need a credit card or even a good team (but the game it a ton more fun with a good team). What you do need to do it take a season off, where you don’t spend any rubies or sigils, and don’t open any chests. Do the best you can in all events, but don’t open anything.
Near the end of the season, dump all the sigils you saved into the festive line. Keep buying the draconic chest at the end of that line.
At the start of the following season, and probably during the first PVP event, open all the draconic, gold, and bronze you saved from the previous season. Complete both 50% discount award lines during the first two weeks, and you’ll be pretty much assured of a mythic that season (as long as you keep doing your best in events).