So, a marathoner and a herbalist are unbeatable once herbalist gets around 10 attack, after that you can increase attack of marathoner once or more to not get stalemate defeat. You can sprinkle some bigshot on top for that sweet-sweet extra draw, just don't make him evasive, it may or may not break stuff after he gets mild again.
How to solve? Maybe add healing reduction or enemies which heal when your squad heals (maybe excluding healing from cards), but I'm not a game designer so adding "healing stealing" may or may-not make the players who use marathoner-herbalist combo unknowingly dig their own grave and be frustrated.