|"Just go right at 'em" - Captain Aubrey|
The party was originally planning to go after the hobgoblins, at which point the tribe would have sent a runner to warn Obmi, who would have hooked around with his gnoll squad and a couple of other friends to block their retreat. Even with the aid of the Knights of Antonius, a group of holy warriors who were helping them out, this would have been serious Surtrouble for the Muleteers. But then the voice of morality spoke up in the form of Freya, the hermit, who reminded them of their duty to rescue the Hand.
In their reconnaissance the Knights had found a couple of passages forking off with the intersection marked with the dwarven rune for "O." It was there that dungeon doctrine was again ignored, and the Knights and Muleteers split up, each having one passage to search. Luck, too, came into it as the Muleteers picked the one that would lead them straight to Obmi's lab and lair.
2. Tactical surprise. Ordinarily on their way to Obmi's lair the Mules would have run across a small group of orcs, all that remained of the once mighty Grinning Skull tribe, who had be set by Obmi to patrol the maze. However, at the very same time, the goblin runner from the hobgoblins had been banging on one of the one-way doors into the maze, and the orcs were escorting him back to the door of the lair, which a gnoll guard opened.
Just then Titus the gnome and self-styled muleborne knight decided to try to sneak down the corridor behind them, wearing metal armor,and thus failing. "Hey!" The orcs swiveled around and everyone rushed forward, led by Titus, who started incanting the syllables of his Choke spell... only to fail and cast a different random spell of the same level instead at the targets (he must have mispronounced Choke as Shock) ... the most fortuitous Lightning Bolt. Bouncing around in the confined space, the bolt fried all the humanoids and miraculously stopped just short of hitting the caster.
The path to the huge lair room was now clear and everyone rushed in as fast as they could. Five gnolls were at various places in the room, Obmi was over by the wall tormenting one of the Hand party captives, who were all strapped and locked into various devices and tables. A huge swiveling brass machine with a pointy end was installed in the middle of the room. Bort the fighter, running to engage Obmi, placed himself in a position to fight the six remaining gnolls as they tried to come out of their adjacent barracks room. This was a crucial if unwitting decision that gave the party tactical control of the room.
With a few good decisions and strokes of luck the party had given themselves a huge positional advantage, which was to widen when Obmi, acting out a tragic flaw, chose to use his invisibility ring and boots of speed not to get away, but to make it to his pride and joy, the repulsor ray machine in the middle of the room. (This flaw was activated by some unusually high morale rolls I threw for Obmi.) The beam pushed back a column of party reinforcements as they tried to enter the room, but the energy wizard Orbit managed to get off a Shatter spell that blew a hose on the contraption. With most of the gnolls in the lair now dead, and the rest hemmed into their barracks, the party swarmed around the dwarf, cutting off his escape and eventually finishing him.
I could have further ruled that the invisibility and boots of speed would allow Obmi to slip past engaging enemies, but the result felt like a just reward for audacious action, phenomenal luck, and the folly of the usually slippery villain. Things would have been very different if Obmi had been shielded by a swarm of gnolls and able to pound the front line with his returnng hammer. What I observed years ago was borne out that day: the advantage of surprise is not always to the home team.