I think you may be making your life harder by thinking of the medicinal form of Tai Chi practiced by most people today rather than going far enough in history to draw on the actual origin of Tai Chi as a fighting art. Being a practitioner of Tai Chi does not preclude one from striking, just as being a practitioner of Aikido does not prevent the aikidoka from striking an opponent in combat.
Here's a little info that could help you out of your bind (from http://www.shenwu.com/taichi.htm
"Complete Tai ji Quan arts include basic exercises, stance keeping (Zhan Zhuang), repetitive single movement training, linked form training, power training (exercises which train the ability to issue energy in a ballistic pulse), weapons training (which includes straight sword, broadsword, staff and spear), technique training and various two person exercises and drills (including "pushhands" sensitivity drills). A hallmark of most styles of Tai Ji Quan is that the movements in the forms are done quite slowly, with one posture flowing into the next without interruption. Some forms (the old Chen forms for example) alternate between slow motion and explosive movements. Other styles divide the training into forms which are done slowly at an even tempo and separate forms which are performed at a more vigorous pace. The goal of moving slowly is to insure correct attention is paid to proper body mechanics and the maintenance of the prerequisite relaxation. Training exercises can be divided into two broad categories: solo exercises, and drills which require a partner.
The ability to "stick, adhere, continue and follow (zhan, nian, lian, sui)" is vital to the application of Tai Ji Quan combat techniques, the majority of which are grappling oriented. Techniques that include pushing, pulling, wrapping, bumping, sweeping, locking, knocking down and throwing (grappling arts) far outnumber striking and kicking techniques. Solo forms training is designed to develop the ability to control oneself; paired practice trains the practitioner to apply the power developed during solo training to another in the most efficient manner.
Modified forms of Tai Ji Quan for health have become popular worldwide in recent times because the benefits of training have been found to be very conducive to calming the mind, relaxing the body, relieving stress, and improving one's health in general. However, it is important to realize that all traditional systems of Tai Ji Quan were originally created for a single purpose, training the practitioner to fight.
Chen Wang Ting's original form of Chen style Tai Ji Quan is often referred to as the "Old Frame" (Lao Jia) and its second form as "Cannon Fist" (Pao Chui). In the latter part of the Eighteenth century, a fifth generation descendant of Chen Wang Ting, Chen You Ben simplified the original forms into sets which have come to be known as the "New Style" (Xin Jia). Chen You Ben's nephew, Chen Ching Ping, created a variation of the New Style which is known as the "Small Frame" (Xiao Jia) or "Zhao Bao" form. All of these styles have survived to the present.
Smooth flowing movements punctuated by explosive strikes and kicks characterize the Old Frame. There are many vigorous movements that involve jumping, spinning while in the air, stamping the feet, and dropping the body low to the ground. Technically, in addition to the explosive strikes and throws, the Chen style contains a great number of Chin Na (joint locking and leverage) techniques. These techniques are a remnant of the original weapons disarms popular on the ancient battlefields, and reflect the warrior background of Chen Wang Ting, creator of the style."
There's much more info on the site. What this suggests is that you can use the concepts proposed by Retro while including a selection of offensive techniques without going against character's the Tai Chi Quan background.
In fact, it could add to your character's backstory. He/she could have spent years tracking down one of the true practitioners of the old style and (let imagination run free here!)