- Estimated achievement difficulty: 3/10 [Achievement Difficulty Rating]
- Offline: 30 
- Approximate amount of time to 1000: 12-16 Hours [Estimated Time to 100%]
- Minimum number of playthroughs needed: 1 [using save copy technique]
- Missable achievements: Yes [Flower Powered (10G), Unstoppable (100G), Second thoughts (50G), Don't look back (70G)]
- Does difficulty affect achievements: No
- Unobtainable/glitched achievements: No
Missables and Endings:
There are four missable achievements in the game. The first you'll need to watch out for is Unstoppable (100G) which requires you to never get KO'd, where both members of your team die in battle at the same time. You can have one die and finish a battle with them dead and that's fine, but if both fall at the same time you'll void this achievement - assuming you continue on and save. Simply dashboard and quit the game, then reload to ensure you don't miss this.
The next three are tied to the game's two endings. When you get to the end of the game and need to make the big decision regarding the flow bridge (it's super obvious but I'll mention it in more detail below), you'll want to quit to the main menu and make a copy of your save. This is done in-game, so no need to get a USB stick or anything fancy. Just choose to copy your existing save into a new slot and then choose not to sever the bridge for Second thoughts (50G). Load up the copy save you made and choose to sever the bridge to get Don't look back (70G) and Flower Powered (10G).
As you play through the game, make sure you don't ever let both characters die at the same time in battle as mentioned above. You can play on the easiest difficulty setting to assist with this. While going through the story there are a number of things you'll want to do before the big decision at the end. The main task is to completely eliminate the rust on each island, with the secondary task of repairing the Nest to full function. While exploring for those tasks, be sure to gather as many indredients and seeds as you can, and return to the Nest/Camp often to cook and sleep to increase your relationship status. This will offer a number of unique cutscenes as well as serve as the game's leveling mechanic to increase your stats.
In battle there are a few unique miscellaneous achievements, there are a few exploration-related achievements on some of the islands, and a few optional bosses to defeat as well. With how open this game is as far as the story and exploration go, it's hard to give a specific "path" to getting everything done. I'd suggest just playing the story naturally and then once you get toward the end of the story, read up on each of the achievements you may have not already done naturally and knock them out one by one. You can use this map to find all of the Nest parts and where some of the specific achievements need to be done.
The "point of no return" in the story for the two endings is fairly obvious, you'll have to choose to pilot the Nest after fully repairing it. Make sure you have all the rust cleaned from the world, all combat and optional bosses completed, and any exploration achievements done before doing so. You should only have the three achievements tied to the endings and the one for beating the game without getting KO'd left when you choose to pilot the Nest. Again, make sure you make a copy of your save before piloting the nest so you can get both endings.