The demons in control of Jo's head had quite a bit of knowledge of magic, so even though she didn't have the talent, there was more than enough that could be done with the right supplies. Certain spells had been set up around the vicinity of the gate that she was at. Which meant that there was an alert as soon as Sam neared the gate.
The petite blonde slipped into the woods, tranquilizer gun in hand. She took aim and fired.
It would be easier to overpower them this way. If they had to, they would use the familiarity with this body to lull the Winchesters into a sense of false security, but using tranq darts and firing from a hidden vantage point was the easiest and neatest way to handle things.