The thing with sue's feat
-as already been mentioned, only real reason she manage to kill him was due to a loophole
-It was a sneak attack in Exitar's brain. The story goes as follow:
1. Exitar kills the head of the watchers
2. Sue enters the body
3. Exitar and his immune system let her pass purposely till she reached the "brain".
4. Sue then had the change of mind when she reaches the "brain" and attacks it after seeing the head Watcher in E's head.
-It is stated both Exitar and the head Watcher will return still someday.
No way, she could accomplish that if Exitar was really trying around that time and didn't let her pass in his body. Of course, it still reeks of PIS.