This game isn't "hard", it's just a test of your patience and how long you want to force yourself into playing this trash game. Almost every mission plays the same with some exceptions, when you are only the commando or a small squad finding a base or building a base from start to finish every single game.
Almost every mission on every medal takes like a hour, give or take depending on your skill and luck if everything goes right and if you save often.
You can basically spam Zone Troopers and Mammoth Tanks if you have the chance for it, and build the Ion Cannon even when it takes 7 minutes to load, it's worth it.
You should save before and after going for a ribbon, and save a lot in general when you feel safe. Saves a lot of time when suddenly your base gets destroyed or army dies for strange reasons.
i personally would recommend watching videos about every mission before you start them, sadly there aren't (good) xbox versions of the walkthroughs, which is sad since a lot of PC videos are hard to recreate and they're always different from the xbox version.
Typing "Tiberium wars gdi (mission name) hard" gives you some good videos, mostly from a guy called "Hard Difficulty Walkthrough". It's better to pick his videos from 4 years ago, since most of his videos from 2 years ago aren't working well on the xbox. It also helps watching other people doing the missions which gives you another point of view on how to beat the missions.
Here's a playlist if you like: https://www.youtube.com/playlist?list=PLnCGGadjNmJzuwQCsUSRb...
And here's a text based version of the missions and walkthroughs starting by mission 1: https://cnc.fandom.com/wiki/North_Carolina_Badlands_(mission
For the Ground Zero Mission you can use the evil tiberium bomb, and still get the achievements, you have to keep rebuilding your base due to nuclear bombing and black holes, even when i did the second ribbon it only showed me 3 out of 4 done but i still got the achievements so don't worry about it too much.