Actually, Russia tends to be bad when fighting wars in the Far East due to having to march huge armies across Siberia, a process which takes a while and inflicts attrition on their troops. So attack from Japan but don't advance into Russia, stay on the defensive to lure their army to you. While you use the Far East as a diversion, consider invading them by sea by landing some troops right into their capital St-Petersburg, then occupy whatever states you want to take from them, and if you win many battles you should have a warscore of at least 20% or more. Impose a naval blockade on Russia from all sides if you can. Just be careful that once you take some Russian cores, they will likely declare war on you every 5 years in an attempt to retake them, so consider allying with Germany as they can tie up most of Russia's armies.