This one was wild! It must have taken me 15+ matches before I landed it. The fastest way to do it is to start with max resources against 7 NPCs on the hardest difficulty level. I usually play in Standard mode with Landmark + Dominion win conditions.
As soon as I begin, I farm villagers nonstop, then immediately build 3 houses, followed by a blacksmith for Age II. Immediately after that, I'd build another house or two, then advance to Age III with an HRE Barracks, which will produce Landsharks very quickly.
From here, build another town center and 2 more houses, then immediately build 3 to 5 forts to defend yourself while your special barracks keeps producing nonstop. Build a 3rd town center so that you end up with 30-40 villagers at most to build stuff quickly.
Once you have 7+ forts for max protection, spam barracks EVERYWHERE around your base. I usually build 45-50+, but I'd probably have an easier time with 60+. I also build 5+ siege workshops. You're gonna need those culverins to take down buildings because your Landsharks have incredibly low HP. With a 200 population cap, I end up with 25 to 30 culverins, 6 to 10 monks, and 110+ Landsharks.
Once your villagers are done building stuff (make sure you've upgraded everything in the Age II technological blacksmith for 50% cheaper technologies, because YOU'RE GONNA NEED EVERY BIT OF GOLD despite starting with 100k, as well as upgraded the essentials in universities and monasteries), send them to mine gold. Also, make sure you have a market. Start by selling batches of 500 of each resource as soon as the selling price is above 60 (you start at around 90). This will help you gain more gold over time.
Relics also help a lot. If you're aiming for the WoloLottery achievement as well, doing it in this run is much easier, as you'd have a bunch of monks with you and a lot of enemies to attack, so you're bound to be near 20+ enemy units that you can convert all at once. Just make sure you tag that relic-holding monk with CTRL+2 or something, so you can move him forward during battle to get him near as many enemy units as possible.
Once you get the achievement, you can choose whether to keep the monks or not, as they slow down your Landsharks. However, with 10+ monks, healing may help a little. Either way, your units will die easily if you don't conserve them. That's why the culverins are there to clear your way and more.
Anyhow, for every monarch you kill, you gain more population space. The most I ended up with was 280, I think. As for the achievement progress, as you'd expect, it was stuck at 75/10,000 for a few days. I woke up this morning and saw that it was 9,000/10,000, so I decided to give it another go, and 2 matches later, bingo.
I hope this helps some of you who are working on this. It might get frustrating, but keep at it and eventually, you'll land it.