1) Skill level in elements is very, very low. It's solely dependent on cards owned and cards drawn.
2) Since more luck is involved than skill, ELO just does not fit anyway. ELO is meant to show skill level, not luck level.
Suggestion
Find a better rating system. On top of that, have a stupid high amount of games, like 100, be the goal for the players, and for every game you are below that number, you lose a certain number of points. After 100 you do not gain any more points, so that is your official score, and skill level. Or even better, games after 100 cause you to gain or lose only 1 point depending if you win or lose.
And make sure official rules are updated accordingly to match any announcements that have new stipulations added in after league starts.