Trying to answer all the questions (apologies if I missed one!)
Yes, I was being a little vague partially because 3 of my 5 players have a reasonable chance of seeing this thread (though they pretty much know most details, just not the details about the miniboss other than a general idea). And also, partially to spare everyone a long-winded story about the miniboss. But, in order to answer some of the questions that were brought up ....
My players (D,J,M,E,and K), stop reading just in case! I know you're grown-up enough not to try and let this affect your actions, but still, just stop reading.
Background
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In very very basic terms, there is a powerful relic and trapped inside is the miniboss. While inside, the miniboss is tapping in to the relic's power and doing bad things.
The only way the PCs have found to destroy the relic would also free the miniboss trapped inside.
When they found the way to do this, I told them in game (by way of the NPC that gave the information) that "... it would be unleashing a very strong foe, the likes of which is unknown but power that they have not yet seen, as fearsome as some dragons of legend..." And, later, I told them out of game during a casual conversation that it is a very high difficulty creature that can kick their collective "rear ends" with but a gesture.
Thus, they are getting the king's men to come help them, hoping it will be enough.
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About the miniboss
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The party is currently 5th level.
The miniboss (as originally conceptualized) is (I don't want to say the exact number) .... let's just say in the teens level). Generally, I have been okay with upping or downing a creature by a level or two from my original concept depending on the level or the party to keep things proportionally appropriate. However, deflating from "teens" to something for the 5th level party just seems like a huge deflate to me.
The miniboss is basically in the relic so he will pretty much 'be there' once the PCs go to it and do the thing
i will not have a hard time giving this miniboss some minions/cannon fodder.
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Some options I've picked up from this thread so far ...
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* let the players run the royal guardsmen, and it turns in to an all out assault on the miniboss (in my particular situation, that might have mixed results, some of the players are just really attached to their PCs and running other things is outside their interest -- the idea of running a temp npc for a session came up once before and it had some player disinterest since it wasn't their own pc which is how i know. But it is an option that i like none the less so i want to still consider it)
* let the royal guardsmen focus on the miniboss while the PCs focus on the cannon fodder
* let the royal guardsmen focus on the cannonfodder while the PCs focus on the miniboss
* have the miniboss wipe the floor with the guardsmen (because, honestly, the miniboss could do just that, though it might be close...) and let the PCs make the decision to stay or flee (after all, they were warned in character twice whenever it came up and now they get to see it for themselves)
* another option i considered was to simply have the method of relic-destruction fail. but, it would come off as major cheese since this is something they've been wanting to do since they got the thing at the start of the campaign.
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I may be forgetting some random relevant facts, but those are the main points that I can think of in response to the questions asked so far.
The suggestions have been great so far, thanks! If the above sparks any other ideas, by all means, feel free to share. I still have a couple weeks before the session itself.