A Rogue 20 actually stands a fairly glorious chance IF they do not directly engage. A high-level rogue can nickel-and-dime the army, and I don't think they can actually track down the darn guy. How do you take down someone who can A) outrun you, B) Hide so well that you can't spot them on a 20, and C) has a variety of other powerful features that further skew the game in their favor.

Assassin or Mastermind? Sow dissent within their ranks (funnily enough, the assassination tricks actually go to waste). Arcane Trickster? I don't think I need to explain why spells are good. Thief? He gets to take two turns while you are still reeling, and likely has powerful magic items! Scout can ambush all day long. Swashbuckler and Inquisitive get the short stick here, I think, as they are better against single opponents.