Lots and lots of experience! No splinter has a defnitive advantage over another as you reach the higher leagues. 

The way I think of Splinterlands is such: 

Each league (Bronze, Silver, Gold, Diamond and above) are their own little games unto themselves, due to the differing depth and interaction of the game mechanics. 

As it becomes more complex, so do the number of moves. If you think of it like chess - first you learn how the pawn works, then perhaps the knight, bishop... except in Splinterlands the chess pieces are all your cards. 

Then the board is never quite the same - with the differing mana sets there's some monsters that can be used to devasting effect, while others ... not so much. The idea is to adapt to the circumstances. 

Unfortunately, there's no real way to write a formula that just works - the only way to figure out what works and doesn't in Splinterlands is just like playing thousands of games of chess. 

It is a very elegant and sophisticated game with crazy amounts of depth. It'll only get deeper with the release of Chaos Legions!