Ways to get Formidable Fighter as an adult?

AFAIK there is no ambition to become a formidable fighter, only for the first two steps of combat ability.

The marshal cannot get kids to formidable level by assignment. Better to give him another assignment then as his effort is waster after the third tier.
(I last checked this two versions ago, so it may have changed since then.)

You can still get to Formidable by the random event as a child. To go to Formidable you have to be at least 13. The odds are increased if your guardian is Formidable himself, and Diligent OR Ambitious (they don't stack), decreases with lower combat ability and Slothful OR Content. If the guardian has the Duelist trait that also helps.

There are a host of modifiers for the child itself, including traits, culture, and sex. Members of House Dayne have a pretty significant boost. If you wanna see all the modifiers take a loot at the event combat_trait.1 in events\combat_trait_event.txt

There are other random events too where you may be able to get to Formidable, for example bullying another kid and using him for martial practice. I don't know much of the requirements and modifiers for those.

Once you reach adulthood it's harder. The only way I know of is a random event when you win duels, and that event is very, very rare when you are trying to level up to Formidable.

EDIT: I took a look in the game files and the chance of leveling to Formidable after winning a duel appears to be 2%.