I don't play as much Splinterland as much I would like to, but when I do the session gets long because i tend to have losing streaks. This is either due to lack of skill or lack of luck on my part. However, today I had some success by using Life Splinters due to quest. I played three ranked battles - I won first two. It seems that Life Splinter, if you set your cards right, is the most efficient, at least at my level.
However, this might be nothing more than my beginner's impression, so don't take it too seriously, gentle readers.