Genocide can be a hassle, as the population is rarely cooperative, and the gains are never enough to cover the expenses of such vast project. FortunatlyFortunately, the human cruelty is nearly infinteinfinite, and this hasn't stopped many powerfullpowerful leaders and sharp mind to spend countless time and effort on such a despicable purpose.
Here is a list of usefulluseful tactics that have been used through history, or that have been invented by that wicked mind of mine. You can use them both for genocide or to simply break them as a people.
As mentionnedmentioned by Youstay Igo, you could find some sickness they are vulnerable to, and use it against them. You have many ways to do this
This one has been mentionnedmentioned by many other answer, so I won't detail it. Even if torturing some prisonnersprisoners is not that effective against the rest of the population, this cannot make them any good, so carry on. It could even be used as a distraction to boost your own troops moral.
In the opium wars, chineseChinese forces were significantly weakened by the use of opiods, wichwhich were illegally imported from britishBritish colonies (this was, as you can guess, the starting point of the war). Getting Wussname to widely use some sort of adictiveaddictive drug may reveal tricky, but very usefulluseful once you succeed. You may achieve this by using some third party, so they do not suspect the provenance of the drug : manipulate a travelling merchant, a friendly country of your ennemyenemy, or send troops equipedequipped with the drug to a suicide assault. They may found the drug on your soliderssoldiers body and try it.
GerillaGuerrilla warfare
Youstay Igo mentionnedmentioned that too. Ambush, traps, landmine and gunpowder are your friends on this one. Use also your dragons to make raids on their cities with incendiary bombs. Beware thought, as this is their country, they know it better than you do. I'd suggest you burn forests rather than trying to use them for an ambush that could be easily scouted and usedagainstused against you.
EDIT : I almost forgot, but this will also grant you the unobtainium. But you may discover that having some instantaneous communication is far more precious than better weaponeryweaponry. Also, as Conan said, there is nothing in life better than crushing your ennemyenemy.