I managed to get this achievements same way i got the original genocidest, when in the saferoom at the start of the finale, on expert kill all bots and you can either do the next bit on easy or expert. Expert may make it quicker, not to sure. But try get the attention of a boomer by going in and out of the safe room and outside a few times, i usually got one when the first horde comes. when You have one just use an elastic band and just do the LT button, against the door with the boomer puking on you.
The boomer sometimes walks away or stands in the corner and doesnt puke on you, but keep trying to get it right and you will.
Good Luck :)