It's not worth playing for the rewards any more for sure!
I've hit on a strategy of using the summoners/ rulesets that have the highest win rate for each rule set (I've got a list) and then I stack with the cards which have the highest win rates. Then I tweak based on the rule-set combination.
There were some surprises. Failed Summoner for Earth, for example - that's a great card to shove in second. Those no attack monsters really have their place!
And it's quite crude also - more damage, you need to do lots of damage - the card with one of the highest win rates is Mitica headhunter from Earth - it does 6 damage maxed out, ranged and piercing.
It still isn't that interesting playing it though.