The expansion story (Evolution Story) is actually pretty easy, even on Predator (Hard) difficulty. If you turn on the max ammo and unlimited feral power cheats ("UnleashHell" and "Bloodlust," respectively) you can blow through the game in a few hours, running past everyone.
There are only a few parts in the game that are challenging, IMO:
Earlier on in the game, you have to get to a Temple and get past some snipers. This is very frustrating, since the snipers usually will kill you in one shot. Use Feral Vision (Hold Y) to spot them. I think there about 5 snipers in this spot, and the rest of the enemies have smgs. One of the snipers is on the center island, and the others are up higher in fort-like structures.
The next challenging part is near the end of the game. After you escape on the raft, you have to protect that dumbass Doyle from enemies. This part took a few tries to figure out, but stay near Doyle, turn on Feral Vision, and use the sniper rifle on the enemies, this way you won't stick out as much. Throw pipebombs if the enemies get too close, just don't stray too far from that idiot or the enemies will flank him.
One though part that may annoy some people is when you fight the commando (with the rocket launcher) after destroying the pipes in the refinery. The easiest way to do this BY FAR is to jump on top of the elevator he rides down, and when it gets to the bottom, feral attack him twice as soon as he runs out of the elevator. Simple.
The last tough part was right at the end of the game, right after the last part I mentioned. You fight the final boss as he jumps between pillars, but this is hard since you can't regenerate health, and he hits HARD. Use the pipe bombs to blow up all the pillars (it doesn't matter if he's on it) and shoot him as he jumps around the room. He does hit hard so be careful. Once you hurt him enough, he'll jump on the one pillar you couldn't blow up and shoot a hole in the wall. The game will checkpoint here and you'll get your regeneration back. Just go to town his face with dual SMGS and eventually you'll beat him.