Highest my character could defeat with/without Prep?

Hero name: Jet

Powers:

Invulnerability: Pretty much. Unless you put her in a lead lined room, upon which her powers won't work at all. Otherwise she won't die. Even when unconscious she would wake up sooner or later. Even so that don't mean she won't be knocked back by force.

Flight: Requires wearing the cape. Speed is gradual for traveling, but she but can do short dashes of speed with sudden halts.

Strength: Unlimited. Nonetheless she can tire out from getting overheated. She can overheat by lifting a ton or more. After a full 2 minutes of lifting a ton or more she would pass out. Not dead, just unconscious.

So note she is not without weaknesses. Even if you doubt that, just reread the OP. It shows how she can pass out due to overheating. And there are plenty of ways to heat up stuff.

Thing

@lorbo: Too many factors to give a decive answer.