This is an easy easter egg to do and is fun to mess around with. Just follow these steps and impress your friends:

1) Reach level 14 or higher.
2) Go to the sanctuary.
3) Patricia Tannis will offer you a side-quest called Splinter Group ,where you have to assassinate a sewer dwelling quartet of colour-coded mutants.
4) Play through the side mission as instructed.
5) Enter the hideout by pretending to be a pizza man.
6) Lay the pizza on the table and the four Ninja Turtles will come out.
7) Defeat them.
8) Find a room with a TV and a simple switch puzzle.
9) et all the lights to turn green and then activate the TV.
10) After a few seconds, Flinter will appear.
11) When you kill him pick up the rare loot he drops!

Here is a video if you are having any problems: