ANSWER: You cannot use true resurrection to defeat a combative lich. The soul has to be willing to return to its pre-undead state. Liches like vecna and the lich-queen choose to be undead. Were you to cast on them, they would simply say, “thanks, but no thanks. Now die, mortal!” If the soul is an unwilling undead, there is some debate over whether or not the undead form needs to be “killed” first.