Maybe there are some constraints that we just aren't aware of as viewers, but you have to wonder why Cyn didn't summon up multiple J's so that she could have more than one autonomous henchman. We see that J is fairly proficient in one on one combat, which is useful in several situations, but probably less useful in a situation where it's you vs the entire universe.
There are two options as I see it: 1) She cannot create multiple active J's or 2) She simply enjoys seeing J get beat up and killed. I'm gonna guess that #2 is the case because it's funnier.