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[Doug McCrae]

Interesting question. Here are some more options:

1) Have the PCs play the royal guard too, making the die rolls etc.
2) Change the mini-boss so it becomes two separate fights, maybe he has allies or something. Focus only on the part of the fight the PCs are involved in, don't roll any dice for the royal guard, their actions are just handled by description. You can decide if you want the guard to win or lose their part of the battle. I'd have the PCs get to do the exciting part of the fight, squaring off against the actual boss, while the guard go up against his allies.
3) Problems arise with the royal guard. They have a traitor in the ranks, or are called away on more important business elsewhere. Perhaps the king, or the wicked captain in charge of the guard, decide this isn't worth risking his men's lives over. Maybe the king is fickle.
4) The mini-boss has heard about this large force planning to attack him, probably due to the afore-mentioned traitor, and legged it before the PCs arrive. This is quite good because it allows you to delay the final big battle until a story appropriate point and build up more anticipation.

Given the players are looking forward to the mini-boss, but you want to avoid having NPCs do most of the work I'd go with either #2 or your own suggestion to use a weakened mini-boss but have all the royal guard taken out by the magic power of Plot.