You can obtain the optional Guardian Force, Tonberry, as soon as you have access to the Mobile Balamb Garden ship or as soon as you can fly throughout the world. You will need to access the secret Centra Ruins area which is where you can complete the Odin side quest if you choose to, and you can obtain the Tonberry Guardian Force (GF) by defeating the Tonberry King in this area.
The Centra Ruins are located on the southernmost continent towards the center of the group of islands. Use the map below to help you find the exact location:
The Centra Ruins are a large octahedron shape in the center of the dusty plains. They can be somewhat difficult to spot as they are the same color as their surroundings, but they stick out of the ground far enough that you should be able to see them as you pass by.
Save your game before you enter the ruins.
A 20:00 minute timer will start when you enter the ruins but you will not have to worry about this timer while attempting to obtain Tonberry. This timer is a time limit imposed on the side quest to obtain Odin, another optional Guardian Force. If you go over the time limit it will not impact your quest to obtain Tonberry King with one minor exception (included in the note below).
Tonberry King will only appear after you have killed 20 regular Tonberry enemies which are found in the Centra Ruins. In fact, they are the only enemies that you will face in these first few screens of the Centra Ruins.
Each Tonberry has around 20,000 hit points so it can take quite a number of attacks to bring each of them down. Make sure that you have powerful attacks lined up and that you have strong magic junctioned to your Str character stat.
One effective strategy that you can use against the Tonberry enemies is to keep one of your party members at very low HP. This will allow them to make use of their Limit Break repeatedly as long as the Tonberry doesn’t kill them. This works particularly well if you keep Squall at low health to use his Limit Break, Renzokuken, because it does a lot of damage.
Tonberry enemies have two very dangerous moves. The first move is called “Everyone’s Grudge”. The math behind this move is as follows: the number of enemies that the character being hit has killed multiplied by 20. So if Squall has landed the finishing blow on 100 monsters “Everyone’s Grudge” will hit him for 2,000.
The second move is just a simple stabbing motion which is something that a Tonberry will do when they get close enough to a character. This move will instantly kill that character.
The timer does not impact the Tonberry King battle other than if it reaches 0:00 you will automatically be kicked out of the Centra Ruins. This means that if you are in the middle of the battle against Tonberry King you will have to start again.
Consider leaving the Centra Ruins and re-entering the area if your timer is below 10 minutes. This will not impact the Tonberry death count.
Keep a rough count of how many Tonberry enemies you have killed and make sure you save your game after you have killed your 18th or 19th Tonberry. Tonberry King will immediately join the battle as soon as the 20th Tonberry has died with no break in between the two fights.
In Game Description: The king of the Tonberries. Appears to seek revenge for all defeated Tonberries.
The difficulties in defeating the Tonberry King are very similar to those you face when defeating the regular Tonberry enemies except that Tonberry King has a lot more HP and the battle will last a lot longer.
Continue using your Limit Breaks to deal heavy amounts of damage to Tonberry King. You should also use 
Meltdown magic on him as quickly as you can as this will greatly increase the amount of damage done by your regular attacks.
Tonberry King will use a move called “Junk” which will hit each of your characters for a fair bit of damage. You can use Protect magic to reduce the amount of damage that this attack does, but be careful continuing to recast this magic if your characters are dying. You don’t want to get too hung up on defending as you have a lot of attacking to do to take Tonberry King down.
Tonberry King has some powerful magic that you should consider Drawing while you can as well. He has both Curaga and Full-Life magic which you may not be full on and both are very handy when junctioned to defensive character stats.
Tonberry
You will receive the Tonberry Achievement/Trophy once you have defeated the Tonberry King and obtained Tonberry as a Guardian Force.
Tonberry King will automatically join your party as “Tonberry” once he is defeated. You can now leave the Centra Ruins unless you plan to complete the Odin side quest, but you will want to reset the Centra Ruins timer if that is the case.
Royal Crown