There's a couple of ways you can go about it.
If you have some time and money to spare, you can finish removing the control arm from the ball joint stuck in the knuckle, and order yourself a Subaru ball joint puller tool. Snap-On makes one, as well as Company23, and there may be others.
If you have access to an air hammer with a chisel bit, you can also try placing the edge of the bit at a downward angle, on the lip of the ball joint that protrudes when it's in the knuckle and air hammer it out.
With 2 people, one person can place a long pry bar between the top of the control arm and the bottom of the knuckle (where the pinch bolt goes), while the other person swings a BFH to the control arm right by where the ball joint stud goes through. There's a chance you can beat the control arm to hell this way, but a non issue if you're replacing the whole arm.
With any method, it can also help to VERY CAREFULLY spread the knuckle apart, where the pinch bolt goes through, with either a chisel and hammer or air hammer.