To get this trophy, "simply" kick fifty enemies off of trains. There's a couple of things you have to keep in mind while doing this though.
There are but a few places where you can get this. Towards the end of Sequence 1, at the start of Sequence 2, during mission 2 of Sequence 5 or during train heists. I thought it best to simply start Sequence 2 and keep reloading the checkpoint. In Sequence 2: firstly you're forced to cover kill an enemy after whistling. After that, ignore the tutorials and run past the next two enemies, alerting them and moving onto the train top. jump onto the next roof, and after that are two more enemies. Keep on the rooftop and whistle at them. Now you have four enemies you can attack. Once you're done, reload the checkpoint. The first enemy won't be there anymore but it's still three enemies every load and you can start right away (I've seen people recommending Sequence 1 but after trying that there was too big a risk for me that the game would move on to the next segment of the memory).
Kicking people off a train requires a bit of skill. They have to be near death next to the edge of the train, and then you have to use
to kick them off. I noticed how sometimes/often this would not even work and I'd get a regular kill. I started just spamming
since I noticed Evie would kick off enemies every time instead of killing them another way. This turned out to be a mistake.
DO NOT USE
TO FINISH OFF THE ENEMIES. THIS DOES NOT COUNT AS A KICK.
I used that button for over two hours and nothing, I thought the trophy was glitched. Then, just to be sure, I used
... And I noticed how that animation looks much more like a kick than using
, which made it look more like a knee to the chest than a foot. I started over using
and after a short while I got the trophy.
In short: reload Sequence 2, use
when enemies are near death close to an edge to kick them off and reload the checkpoint after dealing with the first few enemies (at the front of the train are only three more, it takes a while to get to them and if you want to return to the first bunch of enemies you have to reload the memory with an unskippable cinematic).