You can only Trance at the point when your Trance gauge fills up. At this point your character will automatically Trance. Your gauge fills up as your character gets hit and the like so there is no way (at this point) to raise it any other way or more quickly. If you want, before you go into this battle run around the area to get your Trance gauge filled most of the way up so that when you enter the battle you can use it sooner. Other than doing this, you'll have to wait.

-DragoonMaster

to defeat the poison chamder with trance you have to level up to at least level nine and use the scond attack on the dyne screan at least twice.