I’ve really appreciated guides that have helped me make heads or tails of where to start, and there are few resources for this very specific acheivement.
Disclaimer: The way I got this achievement uses cheese, you don’t need to use this cheese but it makes it probably 1000% easier.
Chose any ruler in 867
Go to the Byzantines, create a new ruler
Change the ruler to a girl
Change culture to central african
Change faith to any Jainist faith
This means that the Byzantine empire will shatter into itty bitty parts immediately
Chose count Haesteinn, dude’s dummy busted
Chose the decision to “Convert to Local Culture” this makes you French
From there, reset Martial perks and go down the gallant tree, then make sure you go far enough down the Strategist tree to get the siege weapon boost, you should have enough points. I recommend from there going into Learning and picking medicine focus. (For extra cheese instead go into Intrigue for kidnapping shenanigans, although those are iffy on ironman)
Now spam Onagers, these will be your best friends. 2 onagers and 1 bowmen MAA are more than plenty, and pump up the number of Onagers, you’ll be seiging with blazing speed.
Make a priority getting a Military Engineer commander, with this I managed to annihilate Constantinople
Also make sure your Martial is “Training Commanders” makes MAA maintenance cheaper, plus possibility for better knights!
Now book it for Italy, do a country conquest CB on the county of your choice.
After that you have some options, the Byzantines are in a civil war so in the meantime you can do some more conquering and snap up some more land and get more prestige, or you can chose to wait and gather money until the Byzantines collapse.
The Byzantines should have collapsed by now, and 9 times out of 10 I found that Thessalonika was free. Kingdom conquest and take all that land for yourself.
This is when I recommend taking out Byzantium, make sure you have at least 3500 troops, if you don’t buy Mercenaries, then book it for their capital and seige it down. Do this to take the Duchy that belongs in your kingdom.
I recommend moving your capitol to Constantinople, it’s just a really good capitol and it feels cool.
From there clean up the rest of your holdings, you should need to go to war for some isles, these should be quick. Make sure you have all your isles before converting to Catholicism, go back between your lenses and make sure the Isles to your south are controlled by you.
After controlling all of Thessalonika: well you should get the achievement, it’s a tad buggy and you don’t actually need to be Catholic, but you can convert if you want!
Without cheesing it’s a lot more difficult but still totally possible! You just need to spend a lot of time bulliyng Byzantium, which can get tedious but it’s still totally possible! This strategy, while modified, also works for Faster than a Fox, and can be combined with it if you are willing to put off getting Onagers until after controlling Italy!
If you have any questions, or additional ideas of ways to make this easier feel free and let me know!