| Running a sandbox game instead of plot-driven one doesn't have much sense if you change the opponents to be "appropriate challenge". They are as they are - the PCs have to plan their actions carefully. If you modify things behind the scenes, player's choices are no longer meaningful.
I thing Greatwyrm's advice is a good one. Have the boss defeat the guards first, then capture the party if they don't retreat. You may leave some of the guards alive too - if the opponent didn't want to kill the party, he may need the others for something too. | 
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