Actually nen battles can turn anyway you want if he doesn't have intel on enemy. If his enemy was Mizaistom he could just trap him in his courtroom. If he did something wrong(Killua killed a shitton of people) he would get shrekett i think. Mizai literally conjures the courtroom on top of you. IDK if he can dodge it.
But okay let's say it's a battle against a manipulator. Killua doesn't know shit about him. Well - is it safe to approach the manipulator if you do know that he is manipulator. His conditions could be anything like making a sound which killua would hear and then killua would pass out for a moment enough for the manipulator to kill him. If you don't know anything about manipulator you're pretty much buttfucked.
I think all this hype about any of the "fighter" characters is bad. You can't be undefeatable unless you have:>very strong battle ability>ability that gives you intel on people
Even if Killua is very intelligent(and we know that he is) without prior knowldge he can't come up with anything. Conjurers and manipulators AND specialists could fuck him up real bad.