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This can be an absolute pain to unlock.
Contrary to what some other people say, my tip is to do the following cheese ***AS EARLY AS POSSIBLE within your gameplay***, as it won't be as effective on higher levels. The reason being that, while the cheese gives a fixed amount of xp, the higher your level is, the more XP you'll need to level up.

That said, what you're looking for is to keep crafting Adaptive Frames.

To do that, find a planet with Iron & Aluminum within your outpost radius. There are a few dozen guides on how to do that on the internet, but a good starting point would be on Andraphon (moon of planet Sumati on Narion System). As soon as you get there, make at least 5 extractors for each resource and dump all of it in at least 3 storage bins for each.

Now, make an Industrial Workbench and start crafting the Adaptive Frames. Each one will give 1XP, which means a full bar of 99 units will award 99XP. Do this boring-ass process until you're tired, or at least until you reach level 50. Then proceed with the campaign and side missions. Since most enemies are auto-leveled in this game, by the time you're done with the rest of the campaign and side quests, you'll probably be at level 85~90.

Then come back to this and keep spamming that crafting button until you're LVL 100.
PSA: getting from LVL 90 to 100, 99xp at a time, might take you as much time as it took to get from LVL 1 to 50... Thus, why you should do this asap.