Hi there,
if you are reading this you probably want to gain 100% Achievement completion in this game. Or you want to get a quick overview, if you should start it. In whichever case, you came to the right place. While probably all the necessary guides for this game exist, there is no overall completion guide available. Until now. I will link to these guides if I found them useful (from a completely subjective point of view).

First things first. The game itself isn't that hard to complete but to get them all you have to put a quite healthy (or unhealthy :D) bit of man hours into it. I'd guess you'd be able to gain all achievements in about 150h. It took me about 200h (but I am easily distracted though :D). The campaign itself takes about 50h and the rest will be grinding.

If you are worried about missable achievements, do not worry to much. There are only 2 ( 4 respectively) which are missable - see section missable achievements for that. You will be able to get all of the achievements in one playthrough.

Any further questions? No? Then lets go!

There are four missable Achievements you should worry about, while playing through the Campaign.
These Achievements are:

Nose for Treasure
Open 50% of all treasure chests.


I, My, Me, Mine
Open every treasure chest.


Collecting is Half the Battle
Complete 50% of the Bestiary.


Filling Up the Grotesquerie
Complete the entire Bestiary.


There are two great Guides by Steamuser: Jedo and Steamuser: Zioshild which I myself followed while I played through the game.

https://steamproxy.net/sharedfiles/filedetails/?id=264223503
https://steamproxy.net/sharedfiles/filedetails/?id=1777921878
or use this, graphically a little less appealing but nontheless comprehensive guide:

https://gamefaqs.gamespot.com/pc/793808-final-fantasy-iii/faqs/50148

I always chilled on my couch, playing with the controlled and had these guides open on my mobile. Resulted in quite a good gaming experience :=)

Look out for those monsters/chests which are marked as missable in the guides above and make sure to get them all. Otherwise there is not much to look out for. You can follow the guides strictly and have less stress later on or just make a cleaning up run, after your playthrough.

There are a few story related Achievements. The ones listed below, will be unlocked by playing through the game naturally. So just lay back, enjoy the story and keep an eye out for the missable achievements listed in the next section

Gone Adventuring with the Wind
Defeat Djinn and obtain the Wind Crystal.


Some Like It Hot
Defeat Salamander and obtain the Fire Crystal.


Release the Kraken
Defeat Kraken and obtain the Water Crystal.


Quelling the Tremors
Defeath Titan and obtain the Earth Crystal.


Warriors of Light
Defeat the Cloud of Darkness.


While you are further progressing in the story you will most possibly achieve also the gill related achievements:

For a Rainy Day
Save up 50,000 gil.


Who Wants to Be a Gillionaire?
Save up 500,000 gil.


Save up your gill if u want them early, but in my opinion there is no need to.

As soon as you get the Invincible (some of them u can probably unlock earlier if u want to) you have the option to get some more of your precious achievements. Note that you will be able to obtain them all after you completed the game as well.

Glutton for Arms
Obtain every weapon in Eureka, the Forbidden Land.


As soon as you enter Eureka just make sure that u collect every weapon available. Note that you will have to fight a boss for every single one of them. These fights can be, depending on your level, quite challenging. But its worth the risk. They are mostly quite good weapons.

One Sword to Slay Them All
Obtain the Ultima Weapon from the Legendary Smith.


Just follow this brief guide by Steamuser: SpEcIAL

http://steamproxy.net/sharedfiles/filedetails/?id=383189556

You probably want to do this before fighting the cloud of darkness.

Additional Information:
After you obtained the Ultima Weapon the Legendary Smith will give you job items (weapons and armor) when you speak to him and one member of your team has a job level 99 (1 Item per lvl 99 Job). Since they are very powerful they help in harder fights and you have to grind those job levels anyway ;). The Legendary Smith will pop up in various locations. Don't worry about it and just fly to the floating continent and wander between Gysahl (in the choccobo shed) and Healing Copse with the Nautilus (faster).

Also you should be able to obtain:

My Little Esper
Obtain a summonable creature.


No Such Thing as Too Many
Obtain all the summonable creatures.


There are 8 summable creatures total. You can get all of them by simply buying them in the store. I'd recommend buying the "first five" (Escape, Icen, Spark, Heatra and Hyper) for 7000 gil in Replito ( northwest of Saronia) or in Doga's Village (Dive underwater with the nautilus at the southern end of Dalg). The other three can be obtained through fighting. Since you probably want to get the bestiary achievement as well, there is no need to buy them (in Eureka).

The locations are:
Odin: Saronia Catacombs (Be lvl 40 or above!) Note: also u can steal his spear with a thief (quite powerful).
Leviathan: Fly with the Invincible to a lake (with a dark blue spot). It west of the Castle Argus.
Bahamut: Fly with the Invincible over the mountains in the southeast of the Vikings cove.

For further information see the beautiful guide from Steamuser: Sweet Tooth:

http://steamproxy.net/sharedfiles/filedetails/?id=341165475

This will probably be the last entry in you Bestiary. And you will get this achievement on top:

Steely Resolve
Defeat Iron Giant.


The Iron Giant is the hardest boss in the game. I won't discuss any strategies here, since I think part of the fun is figuring it out by yourself. I you get to frustrated there is plenty of information online. But keep in mind that you want to be as close to the max level as you can get to, cause this ******er is challenging. Also getting the job specific equip helps.

To unlock him you have to receive all (4) letters from the Four Old Men and King Alus. In my case it helped to talk to them again (with Luneth for the Four Old Men and with Arc to King Alus).
Then fly with the Nautilus from the westernmost point of the Dalg continent in direction to the floating continent. There should be a dark blue spot (kinda like the one by Leviathan). Dive under water and enter the "???" Dungeon. SAVE! After a few steps there will be a cutscene and the Iron Giant will appear. Prepare!

Okay you have done everything, but there is still one achievement missing?? Then it will be most definitely this one:

When I Grow Up I Want to Be...
Attain level 99 with any job.


Jack of All Trades
Attain level 99 with all jobs for all characters.



And it will be left unachieved for quite some time to come. You remember how long it took to take your first job up to lvl 99? Yes? Well then you might follow this quick calculation:

There are 23 different jobs in the game. And there are 4 permanently playable characters. So this will require you to push 23 * 4 = 92 Jobs to lvl 99. Good Luck.... .

The best strategy to do so is to get in one of the early areas, where the monsters are weak and will not give u any damage. Then block 5 times in a row with every character and give it/them the final blow in the sixth round. This will result in every character gaining one job level.

Rinse

and

Repeat

...

You probably want to watch a series on a spare screen if u are doing this. Probably one with multiple seasons.... .

I hope this guide was helpful to you dear reader, and if so I'd be very happy if you could show your appreciation with a like.

Anything missing? Anything to improve? Contact me and I will see what I can do.