That's not possible. If an element is a strong mono, it automatically means it has certain strengths. Those strengths can always be put into a duo/trio/rainbow, etc. and bring along the best of the element. Since no element has everything, using strengths to complement components in a deck will always yield a stronger deck for a certain purpose.
As for balance, I like it, but complete balance is not a must for a card game. There will always be some elements that are a bit stronger at the moment. This top will usually change depending on released cards and currently strong strategies in the metagame. The source of a player's frustration with the balance regarding this game stems from the fact that change is slow.
If mixing elements was allowed and not heavily restricted (or at all) in events, balance between elements would only matter for flavour. Of course, a lot of events are based on flavour and thematics. It's up to us as players and organizers to balance the events, and I think the developer's role would be to make sure certain -cards- are not overpowered rather than certain -elements-.