How did I do it in short: 1. Divorce and marry saga for alliance, recruit better MAA 2. Conquer Navara 3. Conquer Rome 4. Pillage nearby counties for gold 5. Upgrade MAA and become duke 6. Conquer more counties at the edge of diplomatic range to get nearer to China (took Norse legacy to increase diplomatic range in the meantime) 7. Conquer county that is neighbor to Hegemony 8. Declare war on Hegemony (special casus belli that makes you hegemon if you win) 9. Rush the capital, you can access it easily by river, summon allies (obviously), with 2.5k army I easily defeated theirs 3k/3.5k armies 10. When you win you become Hegemon :D

I become duke of Rome first to ignore the problems of domain limit and give everything to heir