I have the platinum for this game. You can't carry over the dog tags from one save to the other just like the trophy for killing 2000 enemies. That must be done on one save.
The same thing didn't happen to me BUT something else screwed me to stop me from getting the platinum. One of the collectibles was glitched. I had 1 more trophy left and it had to happen. I deleted my save and retried it 3 times always the same glitch. It stopped me from getting my first platinum! But i got my first platinum for Sega Megadrive ultimate collection.
I've already got the platinum. It was very frustrating on HARD and Crushing. I also found it a bit hard on NORMAL. It isn't 3 shots and you're dead. That's only at close range
By the way, its not boring. You have to play it more. I once got 8 million points in one round in the abusement park. You just have to play it more. You guys don't know gaming if it bit you on the ass.