Typically, yoga is percieved as a healing, soothing and relaxing practice.
Yet, it can be very challenging, and poorly practiced can cause as many problems as it repairs.
I wonder, how would I go about using asana only as a tool to relax and heal a beaten up body?
Hi Aaron,
First by making sure you get yourself a good teacher who has the knowledge and patience to work with your specific problems. Secondly, by being kind and loving and patient with your beaten up body. 
I agree with Pandara be kind to yourself, listen to your body and find a good teacher. Yoga is meant to be enjoyed not endured. Sarah
There are many levels here … just as the words yoga and healing stand for many things.
Yoga does not heal, at least not the way is put in your question (if it was a question at all). The living body heals itself, and what asanas migh do is to help this process.
I’d say, our wisdom usually is of much lesser depth than the wisdom manifesting in the vital body, so for any acute illness I’d say bedrest, and appropriate diet is the best course to take, togheter with spiritually inspiring lectures, readings, music. Surely there are cases when this is not enough, but I am guessing one would not adress those cases with asana practice anyway.
I strongly beleive that every person is his/her own medic. The choices we make in our life affect our health. Illness, disease comes not from the outside, but from the inside. One might argue that hereditary or accidental illnesses are not our fault, but that’s the result of our shortsightedness regarding karma. First, we chose the bodies to incarnate in, full package, good and bad togheter, second, we chose the nature of the problems we will face in this life. It does not matter if from a worldly perspective, we will be healed, or not, it matters that we face them. In either case, the person’s life, soul will be affected, the soul will be tested and strenghtened by the process. So even if one apparently fails in facing a ceartin disease, and dies, the soul became stronger in the process. In another life, it will be strong enough and will not need this expereince to happen again.
One mistake modern medicine often falls to is to try to heal in a forced way. The pacient must be left free to face the challenge and only those things are appropriate what aid the self healing process. When you do not let an otherwise healthy organism to face a particular illness, or weakness, than you rob it from the opportunity to develop the strenght to defeat it.
Weaknesses, ilnesses are there so we can grow. Using yoga in this, is equivalent for me with using wisdom. Real healing is seeing beyond the outer signs of an illness, and realizing it’s spiritual meaning. But the depth of self awarness required for this to happen surpasses greatly that accesible to most of us. This is the real illness, and yoga, in it’s all encompassing nature can adress this really well.