Your shoot method is incorrect, it keeps setting the droids to each other's impact value.
I suspect you'd want to subtract the impact from the droid's power instead.
Something along the lines of
private void shoot(Droid first, Droid second) {
second.setPower(second.getPower() - first.getImpact());
}
or somewhat more OO-style:
first.shoot(second);
public void shoot(Droid second) {
second.absorbImpact(getImpact());
}
public void absorbImpact(int impact) {
power -= impact;
}