After the early wars, I tried to build Wealth, but it wasn't very effective. So I whipped some Markets and assigned some Merchants until my income was positive again. Only then I was able to run a couple of Scientists. A Great Merchant in Medieval era helped to boost my research slider for a short period.
It was hell of a long economic recovery. Monarch is so much different from Prince! When I finally got Rifling in 1565 AD, Washington and Saladin already had it and Hannibal soon followed. Lost the Lib rush to Washington in 1210 AD. I didn't even get a chance. On Prince, you wouldn't see such dates very often, but on Monarch it seems to be an unpleasant reality.
So I had no chance but to fight a tech parity (Rifle on Rifle) war against Washington. Assigned most of my cities on Rifles and Cannon, built superior numbers and launched a surprise attack in 1710 AD. Just for a case I reinforced the cities bordering Saladin. It turned out to be a wise move - but only because I was not wise enough to check the diplomacy screen for defensive pacts

Anyway, I managed to hold out against Saladin and after pushing Washington out of the continent, I got him to capitulation in 1830 AD. Made peace with Saladin in 1775 AD - for the moment. During the war, my empire got devastated by the annoying AP "Stop the war against Washington" resolutions. I was stupid to neglect Drama.
After Washington capitulated, Saladin and Hannibal were already nearing Infantry. So I decided to do the same. In 1921 AD when I had built superior numbers of Infantry and Artillery and some Anti-Tanks for countering Hannibal's Tanks, I attacked Hannibal. Pushed him out of the continent and got him to capitulation in 1943 AD by leaving him with one city he took from Asoka.
Then it was time to prepare a war against Saladin. But I saw a Modern Armor

That was going to be a nasty modern war. Although my Power graph looked promising,
I couldn't just play like AI Monty and rely on superior numbers of Infantry and Artillery. I needed a solid air force. Beelined Advanced Flight and spammed Jet Fighters and a smaller number of Bombers and a sizable force of Gunships. As Saladin was nearing a Space Race victory I had no chance but to DoW on him in 1961 AD while being technologically inferior to him.
While my Jet Fighters kept Saladin's Modern Armor and Mech Infantry busy healing, my numerous Infantry, Artillery and Gunships advanced towards his cities. In 1962 AD, Saladin did it - he launched a Tactical Nuke, crippling one of my size-40 stack. He basically nuked his own territory. That's the spot:
Luckily, it only crippled, not killed. Quickly I sent some Gunships and Mech Infantry (which by the time I already had) to protect it. Also, I located my supermedic Cavalry and upgraded him to Super MediCopter. While my crippled stack took cover in a forest near Damascus, the supermedic helped to heal it up. Meanwhile, Saladin was stupid enough to build the UN only for me to control. The first thing I did was to pass the NNPT. No more nukes! I dislike nuclear wars anyway.
After the stack was sufficiently healed, it was time to take Damascus with the help of Jet Fighters and Bombers. A couple of more cities fell. But many Artillery died in the process. You can see the losses here:
Artillery is really effective against Infantry. But against Mech Infantry, especially protective Mech Infantry, they get a little bit overwhelmed.
I wanted to finish off the game before Saladin got his SS ready for launch. So when the chance came, I voted for the Diplomatic victory. In the end, Saladin only missed Genetics:
which prevented him building the SS Stasis Chamber
Later I noticed from the screen that I wasn't far from the Domination victory myself.
My end score sucks badly. I don't remember when was the last time I finished with such a bad score:
I guess the end score is strongly correlated to finish date, but the correlation to game difficulty is very weak or non-existent.
There was one AI that wasn't any threat at all - Asoka. Even on Monarch, I was able to out-tech him. I ended up being at war with him because he had been capitulated to Saladin. He could easily take my ill-defended coastal cities by surprise, but he didn't.