Anything I can do for a purposeful and malicious sniper?

I agree that this is a form of griefing. Problem is, there is no rule against it and CG won't help you. You're both wasting crystals and you're both highly motivated to improve your arena teams as a result of this feud. That's exactly the kind of thing that increases spending. The arenas are designed as a toxic cutthroat environment on purpose. Your only recourse is to join your resident shard chat and rely on strength in numbers. This is precisely why shard chats exist.