I heard for the train one you can do the mission where you save the train, and continue dieing in it, and loading it back up. That works, supposedly

From personal experience, it does!

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For stunts, I recommend printing out a list of all the stunts so that you can more easily keep track of which ones you have and haven't done. It's so much better to have a checklist than trying to follow whatever stunt comes up on the pause screen. Also, make sure that you don't do too many side missions because you'll clear out the enemies making it nearly, if not totally, impossible to do stunts. Lastly, some stunts might be easier with good powers and some might be easier with evil powers so you'll have to plan your stunt accordingly. Of course, luck does come into play in some of the stunts.

Hopefully I'm not just repeating what somebody else might have said in this thread.

Thanks to both of you, I added that to the OP.

I have a list of stunts and tips for how to do them. If you know of a better way for one, I'd be glad to add it to the OP as well. Printing a list out is a great idea though since you can only see one stunt at a time in the game (very frustrating). I recommend doing stunts while you're going along with the story missions; it makes most of them a ton easier.

EDIT: to your edit; I agree that some are easier as good or bad. I think I listed those out in the stunt guide, but I may have missed some.

I need to get back to this game and finish of the platinum. Im missing like 5 shards and 2 stunts. But they're freaking impossible to find/pull off.

Some side missions will plop you into an area where tons of enemies are fighting which makes for a great opportunity to try to get a stunt or two (or maybe even a trophy). Also, if you don't progress in the side mission to where it'll autosave, you can keep killing yourself and retrying the same part over and over again until you get the stunt you're going for. 

I remember retrying the same part of a side mission over and over until I finally got the stunt.

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For stunt 20, I believe the electric cables in the historic district (around the circular buildings) are a good alternative. I know you mentioned using cables, but I'd specifically use those if someone does do that route (if you can indeed do it there, it's been a while since I played the game) because compared to the cables that are ideal for this stunt in the warren, the ones in the historic district aren't above water.

I'll throw this in the OP too. I did most of the side quests alongside the story missions as well, so I don't remember which actuaully helped me more.

I believe they're the good/evil side missions that usually pit a turf war or something between the gangs.

In fact, I specifically recall the one evil side mission I mentioned in my post taking place on the rooftops.

For Stunt 20, it works if you use the railroad tracks too. That's how I first did it because I got fed up of trying to find four next to each other (though I wasn't looking very hard).

And I do remember some of the massive fights, but I can't remember which missions.

Also, specifically for the stunt where you had to shock some guy in the air then thunder drop on him, I just used the basically infinite bad guys in the third district after you power up the last station, but before you recharge the generators. (It's still a blackout at that point and if you don't destroy all the trucks and bad guys on those trucks, you run around forever until you do or you can't start recharging the generators.)

I used the train tracks too, I was just giving the location of a specific set of cables people can use if they wanted.

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About finding all the shards. If you do download a map with the loctions of all the shards, it'd be a good idea to print it out and take a pen and mark each location off as you go. It can be quite boring to go building to building and constantly checking the map, but I highly doubt I would have found all the shards without it. Be careful though as you could easily find yourself accidently making a mistake on the map. While I was really lucky that the last shard I needed was in an area that I was already suspicious of the markings I had made on the map, it turned out that I had marked the location of the missing shard as found and it was by chance that I happened to see it as I was triple checking the area.

Also, some shards can be hard to see on the map you print out (depending on your source, blue shard dots on top of the dark colors of the map) as well as on your mini-map in-game especially shards over water. Lastly, I believe that there is a limit on the distance the shard is away from you and being able to detect it. For example, you can't sense the shard on the highest building in the historic district (near ground zero) from street level. Also, there's one shard that wont appear on your map at all because it's on top of a broken pier that you have to hop from post to post to get (neon district I believe).

Okay, I gotcha. I'll add that in there as well.