I just ran this encounter tonight for my group. Now, my group is small - only three PCs - but they are 3rd level. If I'm understanding correctly, the Yisarn encounter is intended to be run with second-level characters, so this should be a little easier for them than intended for their level.
For the size of my party, I scaled things down a little bit by removing the pit traps and one skeleton. So, the battle was against Yisarn, one skeleton and the spider.
It quickly became clear that this was too much for my little party, and they retreated. They took the teleportation circles out of there (and the lightning damage from a failed Arcana check took away one character's last few hit points, requiring some healing on the far side of the circle before they could continue the retreat.
I had the party go back to Eriyel and explain that they had fought off the goblins but that Yisarn was too much for them. I had her be impressed with their level of dedication to helping the elves (they were down to zero or one surge each), so she agreed to provide them with some supplies (healing potions, a coating that could be applied to a weapon to make it deal radiant damage for a round). She also agreed to send an elf with them (the party has no striker, so I made him a simple Twin Striking ranger).
The next day, the party of four went back to the lab. This time, Yisarn was expecting them. He had three skeletons this time plus the spider and himself (no pit traps), but the party knew this would be their only battle for the day, so it was daily powers time.
This was a very, very hard battle for this party, but they ultimately won the day. I did change Yisarn's Necrotic Burst power into a recharge power, and I ultimately used it just twice - once when he was first hit in melee and then later when a melee hit bloodied him. I stopped checking for the recharge - it was just too brutal, even once per round.
The battle did end up with a nice flow to it, though. I used the whole lab area - both rooms - with the skeletons set up around the teleportation circles and Yisarn and the spider in the lower chamber, attacking from range. There was a step by step process of kill a couple of skeletons, kill the spider, go after Yisarn.
I had my cleric make a Religion check to realize that Yisarn was controlling the skeletons and that if they could drop him, the skeletons would be no more. He made it hard for them to get to him, of course, but they were able to at least ignore one skeleton to go after the boss.
In the end, the killing blow against Yisarn was struck by the cleric while she had just 4 hit points and two of her three allies were unconscious. It was a thrilling victory - but man, was it hard!
I like the way the necrotic burst played out as an encounter power that recharges when Yisarn is bloodied. That's how I'd use it if I ran this battle again in the future.