Hello, hello! Have you ever wondered how to annihilate a child? No? Yes? It doesn't matter, you're going to find out. There's no way out.
A text version of the strategy can be found below.
Step 1: Make sure T. Freddy, T. Bonnie and T. Chica don't have any weapons equipped. This is to make them less of a threat in the situation where Giygas B.B. ends up inflicting them with High. The only party member that should keep their weapon is Mangle, with it not really mattering if it's the Plastic Hook E or the Real Hook. As long as it can damage Giygas B.B. it should work.
Step 2: Engage the boss.
Step 3: Spam X-Large Pizzas when you only have T. Freddy in the party. This will make it much more likely that you will end up surviving the first part of this fight. Four X-Large Pizzas should work perfectly fine for this part.
Step 4: Once T. Bonnie ends up entering the party, begin spamming Large Pizzas with the both of them. This is to lower the chances of a really unlucky double Spread Bomb absolutely wiping out the party.
Step 5: Once T. Chica enters the party then the real strategy may actually begin. It is a simple one (but one that might also end up taking a while if you're unlucky). Here is what you should do with each party member during this step:
T. Freddy: Keep guarding just in case Giygas B.B. ends up targeting him. Him taking hits for the party is pretty much the best he can do, although if another party member ends up dying then it is obviously recommended to have him use up a Cake.
T. Bonnie: Spam Status Bomb. That's really it, he does not use any items for the most part, only guarding for TP or using Status Bomb. With this strategy we are specifically hoping for Status Bomb to end up inflicting the Poison state, since this is what the strategy pretty much comes down to, but Blind and High are both also very useful states to inflict.
T. Chica: On permanent item/healing duty. It is recommended to keep everyone in the party above a thousand LP, since otherwise an unlucky Spread Bomb can end up dealing major damage. If there is no one to heal then simply just guard with her to avoid taking any big damage.
Mangle (once she ends up joining the party): Also on permanent item duty for most of the battle. It may feel tempting to use Paravolt with her but for the sake of this strategy it is highly not recommended, especially since Giygas B.B. attacking T. Bonnie can be extremely useful for gaining quicker access to Status Bomb.
Step 6: Keep hoping for Poison. This is the most important step and the part that will take the longest, all you have to really do is just follow what I wrote for each party member above. It should take at least three Poison states before the LP of Giygas B.B. is low enough for the final step. It is recommended to either get his LP Bar to either be completely empty or only have a tiny bit of color remaining before moving on to the final step.
Step 7: Use Mangle's Electroshock skill to finally finish him off. Depending on how low his LP ended up getting (it will most likely be very close to 10% LP remaining) it should only really take 1-3 turns just using Electroshock before he ends up dying.
