For the purpose of immediate clarity, there won't be any detail involving the overall plot of The Last of Us in this write-up. This will, instead, be entirely reserved for the Platinum Difficulty review - as you'd normally expect for any game with a heavy story-driven experience.
It's a fairly rare occurrence to discuss the playthrough of a game whereby the actual plot itself isn't referenced, so I'll approach this purely from the perspective that assumes you have personal experience of playing through the story.
Ideally, I had it in my head that I wanted to be able to review the Platinum difficulty and Grounded Mode within the same post, in order to make this as seamless as possible, but I'm still going to need practically one more entire playthrough of this game in order to achieve the Platinum trophy, and I can't bring myself to follow that up so quickly after beating Grounded Mode, so this version of events will have to suffice.
It's perhaps just as well then that this DLC pack only contains 2 trophies, both related to the "Grounded" difficulty mode - which was patched into the game post-release, and topped off with 2 additional trophies.
The 2 trophies in question are simple by their requirements. You'll need to beat the normal Last of Us story on Grounded mode for one of them, and then beat it again on New Game + for the other.
Firstly, for maximum disclosure, I didn't beat this game twice in order to unlock both of these trophies. Having previously completed this game on two occasions, I was grateful for a shortcut I researched and was happy to call upon this by (sort of) beating the game just once and earning both trophies simultaneously.
To explain further, if you've already beaten the game on New Game + (which I had), you can complete the game in reverse, replaying the entire game via chapter select and choosing the Grounded difficulty as you go through each one. You will have to make sure you leave the Epilogue until last though - This is critical to this method.
The logic behind playing the game in reverse chronological order is that you don't need to go through the game twice in order to unlock both trophies - The reason this works is because you're playing the game through again with a New Game + save file, which will allow you to stack both trophies with a single playthrough when you've completed all chapters on Grounded difficulty, and then end by playing the Epilogue last, where trophies are usually triggered.
Further to this, one of the key challenges within Grounded Mode is that ammunition and equipment is meant to be extremely scarce, and that this is a large part of the challenge. By starting from the end, you'll begin the chapter with your inventory you had at the end of your latest playthrough, which also includes all your weaponry, and this is a big help. However, even with the extra weapons and ammo at my disposal, there are still several segments within this game that gave me a real challenge on Grounded Mode, so I dread to think what a "proper" run would look like.
Other challenges include lack of HUD visibility, so having to gauge health using your own intuition and when to use first aid kits, knowing these also come at a premium on Grounded Mode, is something you need to adapt to quickly, and judgement on when to use health kits could be the difference between life and death. Enemies also hit harder, and give you less time to escape from their clutches too, so expect to die a lot along the way.
On the whole, Grounded Mode is a reasonably solid challenge. My biggest difficulties came from certain parts of chapters where you're faced with alot of enemies and you've got a limited supply of ammunition to deal with them, so you'll need a diverse ranges of tactics to clear these areas, maybe using stealth, melee, crafting bombs/shivs for extra offense or hoping to scramble for additional ammo - all of this required me to learn through trial and error. I hit around 4/5 of these difficulty spikes where I'd spend some amount of extra hours on the game learning, on top of the normal 15-20 hour experience of playing through the game again, albeit in reverse order.
This equates to around a 25-30 hour experience, though I appreciate this may vary, as not everyone will struggle on the same areas as I did, but I would expect a certain level of consistency overall. This method, which allows you to unlock both trophies at once, is a genuine life-saver though.
It would have been rough to have to potentially go through Grounded Mode twice, without the benefit of obtaining your end-game inventory by going in reverse. My motives were purely based around burnout purposes, and I'll only need to play this game once more to finally finish as a consequence of relying on this method. Right now though, that's for another time.
