My plan was simply to make two elements geared for the fight until I reach Champion League.

That's a great strategy to use until silve/gold, but later on less elements are available to use. But with dual you'll have like 4 elements to use.

take advantage of all the resource available

100% True, and about the rental cards, some of mine were canceled like baakjira and I had to rent a lower level :/

One thing that would interesting to know is how much value the rented card brought to the users deck...