Just Cause 3

The requirements for reaching 100% are:
Liberating every settlement: 130 of them. I recommend using helicopters with rockets a lot. They will speed up the progress of destroying everything in each settlement.

Collecting all of the collectibles: recommended to do after the liberation of each province since they will show up on your map at that point. Look out for your sonar at the bottom of the screen telling you how far away you are from your next collectible.

Collecting every vehicle: recommended to do before attempting all the challenges since they will require you to use the fastest vehicles. Some of the collectibles will also unlock vehicles. There's also a mod unlocked by completing land vehicle races that will help you locate missing vehicles with the same sonar used for the collectibles. (garage vehicle collector)

Completing every story mission: 25 of them

Completing every challenge: 112 challenges. You will need to get 5 gears on each and every one of them. Wingsuit courses, scrapyard scramble, shooting galleries and crash bombs can all be optained on your first try without any progress made to your character though the others especially the vehicle races are much easier when you have all the gear mods especially Nitrous II and all of the fastest vehicles.

For more details on each specific task I recommend checking the guides of the affiliated achievements. You can check your progress any time by hitting and going on the "PROGRESS" tab.

Self explanatory Achievement but heres a breakdown and some quick tips.

There are X parts to this achievement:
1.) Finish all the main missions in the game, these include all story missions in the main game. There are 25 missions in total to complete.
2.) Liberate every town, outpost, base, ect. across Medici, there are a total of 130.
3.) Finish every single challenge in Medici and achieve 5 gears on every single one. There are 112 in total.
4.) Obtain every collectible in Medici, including, Diravello Tapes (71), Vintage parts (54) as well as lighting every rebel shrine (49), visiting every ancient tomb (23) and finally doing every stunt jump (30).
5.) Unlock every vehicle (47) and weapon (28) from the rebel drop.

Liberating the map, self explanatory, the game explains how to do this. If you are in a chopper that has rockets, you can actually hit outposts from high up and liberate them without ever engaging enemies, this can be useful later on Insula Striate as military presence is strong and can save you time than engaging on ground. You can't do with with bases as getting close you will get a warning to turn around, towns can be done but only for the destructibles, you can hit towns quick and come back later and do whatever is left. Also, if you have the bavarium sea heist DLC, complete the first mission to unlock the Loochador as this boat has self guided missiles and can make dispatching bases by the sea and oil rigs pretty simple.

I would recommend doing challenges as you unlock them, otherwise they can get overwhelming later on as there is A LOT of them. To make it suck a little bit more most of them will be wingsuit courses which can be easy or hard, there is no in between on those. Basically what I did was liberate a province, do all the challenges as I went along, once a province is liberated you will see question marks appear on your map, those are collectibles. I got those when they were unlocked on the map. I would then continue this cycle doing a mission every so often to keep the game moving. You will be able to do almost every challenge on Insula Fonte as they are mostly easy, the exception will be sea and plane races as these will be easier done later on, the plane races are best done with the CS7 Thunderhawk once it is unlocked when Griphon military base is liberated, air nitrous II will help considerably. For the boat races, the Pescespada SS is the best vehicle IMO, some people say it's the Squalo X7 but the P SS is available sooner and once nitrous II is obtained most races are done with minimal effort.

Unlock the vehicle finder when you can as you will have to bring a ton of vehicles back to the garage. This will also allow you to know when you are potentially near a rare vehicle, mainly the Autostraad Reisender 7 as it is one of the hardest to come by. (To help it is basically the front part of a semi truck, it looks like a European truck with a flat front.) The sooner you can round up the common cars the better chance you will have to spotting a rare vehicle as the finer will not be going off constantly. The Weimaraner W3 is another rare vehicle but it has a spawn point. Look for N40 48.878 E5 37.961, it is the lower of the 2 islands. Believe it or not 2 of these spawn there, just us a helicopter and tethers to get it to the main land and voila. The Autostraad R7 is sometimes found in chance encounters on the side of the road, there will be a trailer with no truck, but rare is when there is a truck, you can also find the Mugello Farina Duo on one of these trailers randomly as well, IF and it's a rare if chance, you happen to come upon the Mugello F D AND the Autostraad R7, please PLEASE take the Autostraad R 7 as the Mugello F D can be obtained from a vehicle race in Regano. The rest of the uncommon vehicles can be found under the page for the Caught 'Em All achievement, and the others are unlocked by liberating military bases on the map. Weapons are unlcoked with missions and liberating bases as well so you will get them in when liberating all areas.

Some challenges to note are, Laguna Blast Crash Bomb, located in Sirocco Sud. This challenge is glitched and there is an invisible wall, look up YouTube videos of getting 5 gears, you must take a certain path and hit the rock at a certain spot to make your car airborne to get points for the crash. Took some tries but I managed to get it.
Monte Dracon Tour in Capite Est, this is a hard wingsuit challenge, look up YouTube videos on 5 gears, you can exploit the mountain by taking a certain path and then grappling back up the mountain to double dip points, I got an easy 105k which is more than needed to get 5 gears using this.
All other challenges, you can look up how to 5 gear on YouTube, but again I recommend nitrous II for all race challenges. With any land race that gives you a motorcycle, do yourself a favor and switch to an off road vehicle, less chance of you falling and you can finish it in a car, there is a way to do it on purpose.
Get ready for redoing wingsuit challenges, this goes more toward any one that demands over 50k points as these are extremely lengthy and messing up or getting 2 or more greats, you will have to restart to get 5 gears.

Note that encounters do not count toward this achievement so if you have something like a rare vehicle, don't worry about doing an encounter. Same goes for priests, they do not count toward 100%, and quite frankly I've only used one once for achievement.

Just got this, I don't know playtime but because I was playing through with the intention of 100% from the start so I have to say it took somewhere between 60-70 hours. Hope this helps anyone going for this.

WARNING

there's one glitch that prevents you from reaching a 100%
the bellevia police station can glitch out if you leave it without completing it chaos bar.
Do not leave without doing this, there's no fix