Kennings are compound, descriptive phrases with metaphorical or figurative meaning that stand in for ordinary nouns and proper nouns. They were common in old english and old norse literature and were used to add poetic dimension to terms that were often used in their language, they describe this common things in a new and descriptive way. The epic poem Beowulf is full of kennings and I think is from where the example of the question is taken.
I belive that the term battle-shafts could refer to arrows because the author use many kennings about weapons or gear for the battle using "battle" as a prefix before another noun; and a shaft is a long rode or pole specially the body of a weapon; it could also be a sword but it is used the metaphor "light of battle" to describe it during the story. Coming back to the term of the question and being a shaft rode or pole we can imagine that the author want to describe a pole used for battle and the weapon most likely to be described as a pole I believe is an arrow.
Other kennings with the term battle used in the epic poem of Beowulf are: "battle-gear" to describe armor; "battle-sweat" to mean blood; "light-of-battle" to describe a sword.
And finally other examples of kennings but not using the term battle are: "whale-road" to describe the sea; "sky-candle" meaning the sun: "ring-giver" refering to a king; "sleep-of-the-sword" to describe death and so on.
I hope that the answer is correct and could help you. Regards
