I believe you are referring to this painting, which refers to the pseudo-mythic Yellow Emperor Huangdi battling the Flame Emperor Yandi at the Battle of Banquan. Among other supernatural abilities, the Yellow Emperor was said to have had the ability to tame wild animals and utilize them in battle, including the pixiu, a legendary winged and fanged apotropaic creature associated with wealth. The term pixiu has become associated with the giant panda, although the modern Chinese name for the animal is da xiongmao, or "big bear cat." Somebody more astute with ancient Chinese linguistics might be able to shed some more light on this odd association.

Both George Schaller's book, The Last Panda, as well as Henry Nicholls's book, The Way of the Panda, note that a variety of animals (i.e., the pixiu, the mo, and the zhouyu) depicted in ancient Chinese texts (e.g., Sima Qian's Records of the Grand Historian) may have been misinterpreted as being giant pandas (e.g., descriptions of Asian tapirs may have been misinterpreted as pandas). For this reason, the anecdotal account of China sending live pandas to Japan during the Tang Dynasty as tokens of friendship can not be verified. Nicholls notes that despite Chinese texts and art that date back thousands of years, no illustrations of pandas exist prior to the nineteenth century. Both Schaller and Nichols note that the earliest contact between a Westerner and a panda [pelt] did not happen until 1869, and one of the earliest depictions of pandas followed shortly thereafter. Although there are accounts that flags with an image of a panda were raised between warring ancient tribes to signify a truce, these accounts appear in those ubiquitous N-Things-You-Didn't-Know-About-Pandas! lists and can not be verified. China's engagement in "panda diplomacy" in the twentieth century is the closet you will get to the idea of pandas actually used in "battle." This is especially true in the last few years, when China has refused to loan pandas to [or require their return from] nations believed to be acting against Chinese interests.

In actuality, your question immediately made me think of the panda from Disney's Mulan (1998), but in order to follow the rules of this subreddit (i.e., re: Jokes and humor), I had to dig out the above information really so I could just post this fantastic video clip.

they had an interesting old painting in the hall where panda's were being used in a battle.

There has to be a picture of this on the internet somewhere, right?