Seeds of Time Hidden Gnome

My cousin and I were playing on the seeds of time map and when we got to the Azia place my cousin was on a wooden bridge to the left as soon as you come through the portal as a zombie and a cloud popped up with a golden gnome on it. I went in on solo play and went to the spot and it didn't appear. So I'm assuming that there is a secret button or something but I can't seem to find it.

I have tried shooting the gong and bell but it didn't do anything. I have an idea that if you shoot all the boards with the zombie sensei it will appear but I am not totally sure if that will work.

For those of you who want to help figure this secret out now is the time. I will keep looking also.