Imagine this is part of a Dungeons & Dragons bestiary.
Name: Balloon-Giantess
Balloon-Giantesses are often found hiding within ancient ruins or large caves. These hideouts are typically found not too far from human settlements. The reason for this is because while they are hostile creature, balloon-giantesses enjoy hording wealth...and men. They like to make grand appearances for the humans to marvel at. The balloon-giantess while inhale vast quantities of air to inflate certain parts of their body. They do this to extract attention and treasure from their male prey. A balloon-giantess will swiftly crush or kidnap a more attractive female human, often boasting of their own beauty and ability to "enhance" themselves before taking any violent action.
Balloon-Giantesses will shy away from a fight that they don't have an obvious advantage in. While huge, they have rather weak skin. Their insides contain air, rather than blood and viscera. This air also acts a sort of "life force" for them, similar to how humans have blood. Their skin is easily punctured by piercing weapons such as arrows, spears, or anything spiky. Puncturing the breasts or buttocks of a balloon-giantess is often a good way to cause her to flee as they can not stand the idea of being deflated. Even brandishing a simple spear will cause them to hesitate. Balloon-Giantesses will often wear makeshift but scanty armor to cover their curves. Your best bet is to remove the armor first In a fight - go for the tits, or ass with something pointy.
To truly defeat one you first must pin it down. Usually ropes or chains will keep one down. Being air-filled creatures, they are rather easy to keep down once felled. Once this is done you can deflate her rather easily. Just keep puncturing her until she is a deflated rubbery heap.
------------------------------------------------
Name: Balloon-Demon
The balloon-demon is a physical manifestation of the psyche of humans with more "unusual" sexual appetites. Balloon-demons have the appearance of human females with a few exceptions, the nipples. The nipples are "tied off" in the same way a balloon is tied off at the end. For this reason, balloon-demons are hard to root out from a city or town. These creatures will seduce men at social gathering places where the men will be too drunk or lusty to realize they are being targeted by a air-filled demoness. Thankfully, the balloon-demon will often get over confident and get themselves popped, so they aren't a huge nuisance
Balloon-demons, like the giantess, are filled with air and it acts as their life force. They will typically avoid sharp objects and will try to leave a room if they spot a needle, unsheathed dagger, or even a table corner that appears too sharp - anything that could "poke" them, they'll avoid.
Unlike the giantess, the balloon-demon will keep most of themselves covered up, except maybe to show some cleavage with some leg. They like to wear corsets and bustiers.
Balloon-demons typically try to avoid combat but will fight or flee when exposed. For this reason it is often best to take them by surprise. If one approaches you, play along and act fooled, when she has you in her bed, drive a needle into her breasts or buttocks! - whichever is biggest, the bigger assets hold more air. In a fight, they aren't very strong, their strength lies in their beauty and ability to adjust the size of their breasts/butt by inhaling/exhaling air.
Balloon-demons often have a "bimbo"-esque personality. While they are good at seduction, they're no match for a man or woman who has their wits about them. Just avoid giving into your lust and you should be ok.