I'm trying to get the Russkaya Pravda achievement and it's been ridiculously brutal. The only time you'll see Slavs are in The Old Gods and the game is seriously stacked against them. They only have tiny little duchies and have to contend against marauding giant Norse armies with unlimited subjugation CBs, other Pagan guys with the same unlimited CBs, and Christians who love to smack them around. Additionally, they have the annoying forced gavelkind like all the other pagans.
I finally had some success and it was pretty hilarious getting it.
I basically started as a tiny two county guy here:
As you can see, in the 30 years or so since I started I did pretty freaking well. If anyone else is trying to get this achievement, here's how I did it:
1. I immediately raised all my troops and attempted to imprison my other vassal count. This obviously made him rebel. I defeated him, and won the war. Then I banished him. After which I did the same to every single baron. I needed the money for the inevitable next step.
2. Even before I finished step 1, as expected, I was declared war upon by four different countries. The AI are like vultures and they jump on weak targets. So if you're a Slav, they will all want a piece of you unless you expand FAST. I also put new vassals in all the empty baronies.
3. I used the 300 gold I got from all the banishing to hire a 225 gold mercenary army. I merged this army with my existing one so my ruler can be a leader. This is important as you need the prestige, tech, and gold from it. This 2250 size army is more than enough to handle 4 countries at that point in time. I defeated the initial invading armies and conquered their capitals, after which I withdrew my armies back to my country. This is because I couldn't afford to maintain my mercenary army to siege those wars to victory. As you can tell, I'm bleeding money and I need more ASAP.
4. I go for the easy targets, my own barons again. I got another few hundred from them and replaced them once more. This time, I also put my extra sons in them too (Look at that picture and see how much of a baby factory my wife is). This is so I can kill them easily afterwards.
5. Now I start declaring war on everyone I can declare war on. I subjugate everyone. After I subjugate them, I immediately try to imprison them again, and banish them for mad money. By now I have hired this same mercenary army for over 20 years. Again, nothing can stand against 2250 men and everything collapses and gives me phat lootz.
6. At this point I turn around and start farming my barons again. I also make sure to kill off my baron extra sons too. I'm quite happy that I haven't died of old age so now the succession is secure even in gavelkind and hopefully the next ruler can reform the faith and get a proper inheritance law.