Find the code

#--------------------------------------------------------------------------
 # * Create Battleback Sprite
 #--------------------------------------------------------------------------
 def create_battleback
	source = $game_temp.background_bitmap
	bitmap = Bitmap.new(640, 480)
	bitmap.stretch_blt(bitmap.rect, source, source.rect)
	bitmap.radial_blur(90, 12)
	@battleback_sprite = Sprite.new(@viewport1)
	@battleback_sprite.bitmap = bitmap
	@battleback_sprite.ox = 320
	@battleback_sprite.oy = 240
	@battleback_sprite.x = 272
	@battleback_sprite.y = 176
	@battleback_sprite.wave_amp = 8
	@battleback_sprite.wave_length = 240
	@battleback_sprite.wave_speed = 120
 end

The changes

Go to where it says

bitmap.radial_blur(90, 12)

Change it to this

bitmap.radial_blur(1, 1)

After you have that go to where it says

@battleback_sprite.wave_amp = 8
@battleback_sprite.wave_length = 240
@battleback_sprite.wave_speed = 120

Change it to this

@battleback_sprite.wave_amp = 0
@battleback_sprite.wave_length = 0
@battleback_sprite.wave_speed = 0

That's it, when you go into battle the background won't have that blur effect