A lot of good information in guides for both achievements already, but they're missing some stuff that I had to figure out on my own. Specifically, how to spot an "Extreme" planet and what to do if your progress isn't counting up.
The two easiest ways I found in spotting an "Extreme" planet were 1. From orbit if it says AGGRESSIVE SENTINELS in red text. or 2. On the planet it will say "EXTREME" above the hazard bar (the one that has either fire, toxic, freeze, poison).
At one point I was trying to finish up this achievement, but noticed my progress wasn't moving up. It turns out, if you die after making some progress, it resets, but the progress counter does not show that. In the case you were at 6.3 sols and die, you will have to redo those 6.3 sols in order for it to catch up and start showing progress again. I thought this might be the case and took a chance on an Aggressive Sentinels planet and left it overnight (I sat in one of those skinny bunkers you usually find in twos, on planets with weather you can also dig into the terrain and make yourself a hole that's deep enough to prevent any hazard damage). Woke up in the morning with both achievements after having already spent an hour there with no progress counting towards the numbers I already had.
If you want to leave this overnight, plug your controller into your xbox and rubberband one of the analog sticks. The important thing here is plugging the controller into the xbox and not just a power source. Plugging into a power source will cause the controller to go idle, even if rubberbanded. Plugging into the xbox will not.
Hope this helps fill in any missing information for those that are having any trouble with this achievement.