So for this achievement you must kill "Hitler" in 7 different ways. Make sure you save before the cut scene leading up to him. I am using the general formatting and such from Maka91 (the creator of the video) so all credit should go to him. I am merely sharing the info I have found. The 7 different ways are,
[1] Just shoot and kill him.
[2] Shoot the bell so it falls on him.
[3] Throw a grenade from the kill.
[4] Kill him by blowing up the barrels.
[5] Shoot him while he is in the car driving away.
[6] Shoot an explosive near where the target "gets away".
[7] Kill him by destroying his vehicle. (while he's in it)
Here is the video:
Also, do note that in the video's description he has made hyperlinks so you can view which ones you need and not watch the whole video. If you guys need any help let me know. Hope it helps!