That's funny, I found HZD a bit too easy on the normal difficulty. There were a few boss fights that where difficult, but not impossibly so.

I think maybe this depends on your play style – I most often went for the strategy of divide and conquer, lure individual machines away from the pack and get them one by one from a distance, usually with a combination of hard point arrows and tear blast arrows to get rid of their most lethal weapons. Whenever I went for using the staff to kill a machine it was too hard, unless it was one of the smaller ones. Also, making use of different ammo to target weaknesses in the machines proved quite effective, and after a little while you just intuitively know which kinda of ammo works against which machines.

Hope you give it another go, I think the story really is worth it!

Edit: definitely second your feelings on RDR2. First rockstar game that had me hooked from start to finish, such a great story. I never played the first one, and I'm a little hesitant to try it because of how great and beautiful RDR2 is, and how well it plays, and I fear playing the previous game will feel too dated in comparison, regardless of how good the story seems to be.