Complex sentence: Just as Lurtz is about to kill Boromir, as he aims his crossbow between his eyes, Aragorn smashes into Lurtz and knocks the bow out of his hands.
Split into simpler sentences: Just as Lurtz is about to kill Boromir as he aims his crossbow in between his eyes. Just in time, Aragorn smashing into Lurtz and knocks the bow out of his hands.