In the rules it clearly states that the fellowship may not take part in an heroic duel. The harder case is for example " The Nine abroad" legendary formation, that hasn´t got it speficially noted that it cant take part in heroic duels.
The fellowship of the ring is trated exactly like an normal company of warriors in a fight. That means that once you cause number of hits equal to their reciliance(3) one member of the formation is removed. It also means that if you cause, for example 8 hits, two of them would be remowed and the two remaining hits would be wasted. Another thing is the fact that the hobbits are treated as a single model for causulaty removal, but thats an separate rule.
That clear enough?(i hope not too many spelling mistakes)