For me, this was a pretty straight forward conquer game where I was almost always at war except to bring back up opinion. Completed it in 3 generations from Rurik. Remember that this is not a "control all land with..." achievement but only requires you to make the Empire of Russia your primary Empire title. I think it was minimum 79 total counties in the empire that you need.
You want to start out go for anyone that's smaller than you to big up your levies. As you are tribal, MaA's cost prestige instead of gold. I went down the diplomacy path to maximize my prestige gain per turn. As you slowly start gaining land, you can create duchy or kingdoms as needed for more prestige. With the gold saved, I bought mercenaries to help with any battles I knew I was going to lose or peasant rabbles that occurred while somewhere else.
For me, the last big hurdle was getting all the land from Khazaria. Eventually you will get all the smaller non-kingdom wars over with and they will be the only remaining nation with significant counties you need. I was roughly the same size as Khazaria so I was able to start a subjugation war and won it, gaining me whatever remaining counties I needed. Took a decent bit of time for the size of their empire but mercenaries help greatly as your armies don't cost any gold.