That or complete a chain quest as a Free Officer.
If the Ruler NPC notices you after your 500 kills, you'd either be offered a Marshal Rank or a Prefect (if that kingdom's hadn't any). If you want a fast marshal uprank, just Join Battles as a Free Officer with the Ruler in your joining side as a participant. You don't really carry over your acquired WoL in your new playthrough, unless that CAW is an NPC.
I know that Flying General gives you a higher chance to be elected as Marshal.
