Once you have a number of cards from xingwang's blog

However, when you hit Dungeoneering, the challenge will be harder, much like the Arena in Hearthstone.  The rewards are better and you need to know the cards well in order to construct a balanced deck https://www.lolga.com.

Once you have a number of cards, you can start to build competitive decks for each of your legends.  It is important to note that each legend has their own win condition.  This needs to be identified if you want to runescape mobile gold build a solid deck for them.