It sounds to me like everyone had fun, so that's the most important thing.
The encounter seems mostly to have been so one sided for a couple of reasons. First, everything seemed to work out for the PCs; every spell and stunning fist worked because saves were failed. The monsters barely got a chance to even really do much. But the dice are the dice and sometimes that's what happens. It could have just as easily worked the other way.
But beyond the dice, it does seem like some of the monster abilities were ignored or not utilized anywhere near to full effect. For instance, the diviner could have likely avoided the stun for himself or the Druid by swapping a die roll out.
Thank you (I'll stop you right there).
Yes, the monk rolled very high on both his initiatives (he went first both against the archdruid, and later the post-forcecaged Death Knight). That's a huge win for the party.
As for the stun, the Diviner (whom I will call Master Thalder from now on, since he was a dragon cultist first and foremost, and only used the Diviner stat block because I liked that one better than, say, Evoker or Conjurer) did use Portent on himself. Or tried to, anyway.
Making a DC 18 Con save is extremely hard for regular humanoids that aren't supernaturally strong.
Even if you have a reroll, and even if you had had advantage (somehow), you still need to roll 18 on one out of three d20 rolls. That still only happens less than 40% of the time.
I think people (that criticize the way my NPCs acted) are
seriously selling player character heroes short in this edition.
To assume a humanoid NPC can resist a Monk stun is to seriously underestimate just how supremely superpowered WotC has made player characters in this edition, compared to monsters and NPCs.
NPCs just don't have the numbers they would need to compete. Take the Blackguard, for instance. Its attack bonus is +8 (I think) while the martial PCs rock at least +11 if not +13. (That archery fighting style is a serious offender in this regard)
NPCs simply does not stand a chance against player characters.
Remember that even if the Druid had resisted the first stun, the monk would just have tried again.
In this particular scenario, he jumps out from close to a hundred feet away, and still manages to get in, what, four attacks if necessary.
So even if the NPC gets three shots at saving against Stunning Fist on every attempt (which is generous), and even if I gave her AC 19
and disadvantaged attacks (which I didn't), chances are
still that she would have ended up stunned when it was clear she lost initiative to the monk.
And there is the explanation for why Master Thalder didn't Portend her stun save. He had already used it to try to gain a better initiative for himself. (She rolled 13 I believe, a roll Thalder didn't dare try to improve. He himself rolled something in the single-digits, and so he rerolled, only to still roll poorly. Not that it would have mattered, since the Monk's initiative of 21 was still almost unbeatable.
Yes, obviously the Monk has the Alert feat. What did you think? 
)
Do the math and check it out for yourselves. The probability of making an +11 attack against AC 19 with disadvantage might sound low, but don't forget the Monk's four fighter levels. His superiority dice completely changes that equation. Adding +1d10 to your attack roll after the fact, I mean.
So I'd say it would be unreasonably optimistic to believe the Monk would not hit her at least twice. (Remember, while Shadowwalk or whatever its called takes a bonus action, he's got Action Surge for emergencies like this, so he can make four attacks even spending his bonus action on something else than Flurry)
And assuming he hits her twice (with disadvantage, against AC 19) she's got a 37,71% chance of making both DC 18 Con saves (with advantage and Portent on both rolls

).
All that speaking to her advantage and she still gets taken out
two out of three times.
I'd say it speaks volumes about
how exceedingly generous WotC has been in giving characters goodies. (Assuming your players know how to use them)
And not giving them to NPCs.
Reminds me of the (bad old) 3E days where the situation was the same. Only then the difference was spelled "magical bonuses".
That is, the NPCs still sucked badly compared to PCs. But to not suck even worse they had to come loaded with magical items.
Which the PCs then promptly looted, making the disparity even greater the next time. Fun times (or not)!