相手 can mean opponent, but it commonly refers to any party involved, "party" being legalese to define any person.
The sentence can be translated as follows:
"As the sparks from the fuse create a loud noise, it will startle anyone when this occurs in front of them."
A bit awkward, but I can't really think up a better turn of speech. Key phrases in this sentence are
Sparks from fuse -> create loud noise, therefore occurs in front of anybody = startled person.