| Leave the mini-boss as-is.
Have the player's play their PC's.
Let the Royal Guard handle the mini-boss.
Give the mini-boss some heretofore unrevealed 'special henchmen' for the PC's to tangle with. Remember that the PC's need to be protagonists, even if they aren't the neccessarily the big damn heroes.
If you go this route, don't make the special-guest henchman too tough... you don't want to give the PC's the impression that their choice to involve the Royal Guard was irrelevant. Doing that should make the encounter easier. The special henchman are more for the surprise factor and to keep the PC's from being purely spectators in the climactic scene. | 
And yet another word from our sponsors
