If the miniboss is truly stuck at this location, whilch is accessible anytime by PCs & allies, obviously he is dead meat. The situation then doesn't sound exciting at all to me, I'd be tempted to say "OK, the Royal Guard kill the miniboss" and get on to something more interesting.
I guess if you want to run the fight, do it as is. At least this could be a good opportunity to show what tough hombres the Royal Guard are, which will greatly increase player satisfaction later in the campaign when they exceed them in power. You could set up some friendly or not so friendly rivalry dynamics, wity haughty, arrogant Guards, a more friendly guard, "Stay back, kids - this is Man's work" and such.
I'd leave the mini-boss stats as is, no reason he needs to be beatable by the PCs alone, and they have done the work to get the Guard to beat him. They should be rewarded for good planning, not have it negated by GM fiat.
Edit: If the PCs can't hit the BBEG at all with his current defenses, there is a case for lowering AC etc & increasing hp. If he's an Elite, make him a Solo of lower level, with the same XP total.
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