When in doubt, change the module to fit your game.
My group had six 5th level characters when they met Nezznar so I boosted the encounter a fair bit. I added a second, 5th level wizard, an extra bugbear, a flesh golem, and an ettin. I know, it sounds way over the top but I expect my players to be ready to overcome overwhelming odds with smart tactics.
The first time the party approached Nezznar's room he sent a Flesh Golem after them while they were fighting the bugbears in the crevasse. The golem arrived just as the last bugbear died. The second wizard had been providing support by sniping with Magic Missile from across the crevasse. When the golem showed up the party was injured and getting low on spells. They nearly panicked but it is a well seasoned group of players so they regrouped and ran away while tossing Fire Bolt and other fire attacks at the golem. As much as it scared them, it didn't land a single blow. That failed assault weakened Nezznar's forces and let the party figure out there were two ways into his room leading to the two pronged attack.
They cleared the rest of the dungeon before returning to Nezznar who had posted bugbear guards to provide warning when the battle was arriving. The second wizard cast Spider Climb on Nezznar who used Invisibility on himself so Nezznar was able to take a good tactical position for firing a Lightning Bolt in the hallway and catching the biggest melee characters in it. A few lucky attacks and well chosen spells meant Nezznar didn't survive long enough to retreat to his room and get behind the ettin but the second wizard was able to use the ettin for cover while casting Magic Missile.
The giant spiders just sat menacingly in their corners and did nothing with Nezznar out of the picture (leaving a few PC's feeling a bit creepy). My added wizard also had a ring of spell absorption (sort of like a ring of spell storing but better for defense). The wizard almost escaped using Misty Step but a couple of party members were circling around to the other door to make a rear assault and he materialized right in front of them.
All that turned the fight into a worthy challenge that left every player unsure if his character would survive.