Here is my 100% Achievement guide and full walkthrough for Tell Me Why (Chapter 2).
This is obtained in the back room of the store, after helping Big Mike count tomato sauce bleach.
He then asks you to count some plushies (There are 11 in there btw when asked).
This is where the massive battle to end all battles begins!!!
You and Mike will be hiding behind some shelves, press and hold
(Depending on where you are) to pop out of and stay out of cover, then when Mike pops out, you have around 3 or 4 seconds to aim your reticle on him and press
to shoot him.
If he goes behind some boxes on the left, you need to go there too, just interact with them to change cover.
As soon as you see him pop up, jump out, aim and shoot, do this 3 times and job is done!
Its extremely easy as not once did he pop out for me.
Still if you prefer a visual, go to 1:24:10 to see how this is done
Hope this helps!