Monarchs cares about power. Give the ruler some more metallurgy or siege engines first, so you have their favour. Then split the Royal Court’s physicians into two groups, one that washes their hands, and one that doesn’t. Do the same for leeches, bloodletting, hydration, etc. It’ll be hard to argue with the resulting death rates. And in the long run, you’ll have a much bigger impact by introducing empricism/A-B testing/evidence-based medicine than any one thing specific thing you could have done.