Technically the actual Guardian Force is just called “Cactuar”. I guess you don’t get to keep the Jumbo version, but that’s okay, as this Guardian Force provide a ton of helpful abilities that will be very useful to the character that equips it. Check out the Cactuar.
Travel to Cactuar Island which can be found on the east side of Centra continent. You probably already know where the location is if you did any Guardian Force leveling using the Earning Fast Ability Points (AP) trick. Check out that section below if you missed it:
Jumbo Cactuar is the little green Cactuar that pops up randomly all over the Island, but Jumbo is a fairly difficult fight, so lets do some preparation before we engage in the fight.
Mug ability from any of your characters. You can steal 
Cactus Thorns from Jumbo Cactuar, but these are fairly ubiquitous, and stealing from the boss will prevent it from having any drops at the end of the fight. In this case, the drop you would be missing out on is a 
Gaea’s Ring, which is just a much better item.
Elem-Atk-J (i.e., to your weapons) as Jumbo Cactuar is weak to water-based attacks. With those steps completed, run around Cactuar Island until you run into the little Cactuar animation that pops up in the sand.
In Game Description: Some Cactuars live long lives and grow huge. This Jumbo Cactuar is one of those.
With the preparations made above, the fight against Jumbo Cactuar should already be made significantly easier. There are two major challenges that you will be dealing with during this battle.
There are some additional hints that can help you take down Jumbo Cactuar more easily. For starters, cast two spells on him: 
Slow and Meltdown. Jumbo Cactuar is susceptible to Slow which will lower the amount of KO’d characters you will have to deal with. Meltdown, which can be drawn from Jumbo Cactuar directly, will lower its defences and make regular attacks hit much harder.
Towards the end of the battle you will see a message pop on on the screen that indicates “Jumbo Cactuar is hesitating…”. This message means that it has fallen below 5% and it is preparing to run away from the battle. Go all out on offence when you see this message and to bring it down. Consider using Aura on Squall and having him line up a Renzokuken or some other powerful attack.
Gaea’s Ring
Cactuar
You will receive the Cactuar Achievement/Trophy once you have defeated Jumbo Cactuar and obtained Cactuar as a Guardian Force.
You’ll obtain “Cactuar” as a new Guardian Force at the end of the fight. Cactuar comes with a number of great abilities. Start off by having it learn the following abilities in this order:
Since you are already at Cactuar Island, it may be tempting to begin leveling up Cactuar using the Earning Quick Ability Points (AP) trick, but if you are following the guide from start to finish and just got ahold of the Ragnarok ship, there are a couple of other Guardian Forces that you should consider picking up before you go leveling any of them.
Use the link below to go back to that section of the walkthrough if that’s where you are at:
Luck+50%