Before meeting the main villain in a final encounter, it's common to have an encounter with a deputy or subordinate, who provides a substantial challenge, but one which falls short of the deadly dangers the heroes will face when they finally meet the Big Bad Guy. Although this concept has been systematised in computer games, it has relevance to pen and paper roleplaying games as well. The challenges throughout an adventure should ramp up in difficulty so that the heroes can gauge their readiness to tackle the main villain. If they have serious difficulty beating the sidekick, then they need to retreat and rethink their plans and resources before they take on the big guy.

At least in theory. What they actually do when faced with such a setback is anyone's guess.

Transcript

Luke: Okay, here we go. I take cover behind the corner.
Luke: R2, go out and draw his fire.
R2-D2: I'm still downloading. Also... how about you draw his fire?
[SFX]: < doop bip boppity-bleep squee boot poppy >
Luke: I'm the one with the blaster.
R2-D2: And I'm the one with the brains.
[SFX]: < doop bwoop >
Luke: Have you got a map yet? Can we double around behind him?
R2-D2: It might be a long load...
[SFX]: < bap-jing boop ziku-ting >
Luke: Well hurry up! We don't have all day.
R2-D2: I don't want to misread.
[SFX]: < doop boop boot >
Luke: Screw it, Vader already knows I'm here. I use Force Sense.
GM: The helmeted guy is gone.
Luke: {looking around corner} Gone?
R2-D2: Maybe he took fright.
[SFX]: < bap-jing doing pip-ting >
Luke: Wow. Worst mini-boss fight ever.