I don't want to seem overly critical here, but winning with Huayna Capac on Monarch with a medium continent and 18 civs is not a reflection of winning on Monarch difficulty. My suggestion to you is to stop playing monarch and start playing prince difficulty with a random leader, and the default number of Civs on a normal continents map. Promise yourself that in the game you won't build any wonders, found any religions, or build any buildings other than granaries, courthouses, forges, libraries and lighthouses. All of your left-over production should go to building military units. Build only one additional city; to grab the nearest military resource. Figure out how to use these units to smash through AI after AI. Trust me, do this on prince and you will find winning to be ridiculously easy, although it might not be as quick as you are used to.