Speech and gestures are important, so the character must have his hands free and be in a position to speak. However, turning is not like spellcasting and is not interrupted if the character is attacked during the attempt.-DMG

Close enough to clearly see the actions of the cleric.. See Encounter distance in the DMG and observation ranges in the players handbook.

Think about the cross and vampire scenarios we see in the movies. Its not that far.

If the mobs are close enough to see the cleric, let im have the 2d6 and see what happens. As dm, just make it fun and dont get stuck on the details so much. Dont get mad if your Dragon gets killed early either. You have an unlimited supply.