If Tyrion called Garlen to the stand, the best he could say is "No, I did not see him poison the chalice." To counter that, Cersei has all her witnesses to the fights that Tyrion had with Joffrey, the threats he's made against Joffrey and Cersei herself, and she even has Lady Merryweather testifying that she did see him reach into the chalice, plus Shae who says she overheard Tyrion and Sansa plot the whole thing. I'm also thinking that, no matter how good a fighter Garlen is, he is no matc