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Boromir is a very high level fighter. He has a very high Con, Str and Dex. His feats included Combat Reflexes, Cleave, and Great Cleave. When he downs one orc with another running close, he gets an extra hit on another one near. All these rules for cleaves and attacks are abstract, so it would not be too difficult to have an extra cleave attack look like just another sword attack.
On another note, I hope you have read the book as well as watched the movie. Boromir in the book had far more arrows stuck in him. I believe he was stuck with around 20 arrows in the book and was breaking them off while fighting. He also downed many more orcs than he did in the film.
Boromir was the greatest warrrior in the Kingdom of Gondor, the greatest human kingdom in all the lands of Middle Earth. Boromir was an epic level fighter capable of fighting prowess second only to possibly Aragorn and Legolas, if that.
Though overall I enjoyed the film, the battle with Boromir rather disappointed me. In the book, the orcs were so fearful of Boromir that they dared not even approach him even while he is barely alive and stuck with enough arrows to kill a 20 men. No orc even thought for one moment to stay behind and risk Boromir rising again for one last killing spree. I felt it was disrespectful to the Boromir character to have Aragorn have to save him. Boromir just didn't need saving, and I had hoped they would have stuck closer to the book. I guess they wanted one more action scene.
Boromir was one of my favorite characters in the book. I wish they could have kept more to the book battle than they did. I wanted to see Boromir cut down so many orcs that they refused to engage him in melee battle. Then about 10 orc archers would have started to pepper him with bow shots causing him to rush and kill more orcs. His horn would have been cleaved by some lucky blow struck by an orc who quickly died. Then they would have taken the hobbits, while Boromir was near dead but still standing glaring at the orcs. They would not have come near him out of fear.
I just hope they do Eowyn properly in the next two movies. She is also one of my favorite characters. I wait to see the scene where she defies the leader of the Nazgul to protect her fallen king. One of my favorite parts in the whole book. Brings tears to my eyes everytime I read it.