My interpretation is that Roy means for Belkar to kill Nale. Between Haley's arrows, Belkar's attacks, and whatever is up next to hit Nale, Roy is trusting on the deafness duration to be long enough to permit that by throwing Nale way off his game. Roy knows that Nale does not react well to surprises and he is milking that for everything here.

Belkar being deaf too is a side-benefit, as it protects him from something the Order knows to be one of Nale's staple spells - Suggestion being used against Belkar's well-known poor Will save. That Nale went ahead and used Suggestion makes Belkar's deafness useful to Roy and plays further into Roy's plans.