It's remarkably easy to get distracted by a summoned draugr when Halldir teleports, because there really isn't anything to visually distinguish him from any other undead in Skyrim. To make matters worse, he's highly resistant to any poisons you're likely to have at low levels, and loves either hanging back and barraging you with fireballs, closing in and electrocuting you, or meeting you in melee with his zweihander. Finally, his semi-random teleports around the room will often take him out of your view completely, and he usually uses these opportunities to summon more draugr to fight you. Try to avoid spending your attacks on these and instead focus on Halldir- he’s not that much tougher than them in terms of defenses and health, and since they’re summoned they not only drop no loot but will be replaced when you kill them.
A few health potions should see you through, even if you're a lightly-armored sneakytype. Slaying Halldir lets you loot him and take his unique weapon, Halldir's Staff. If you're really lucky, he'll drop it when killed and you can both pick it up off the floor and loot it from his ashes. Halldir's Staff has the unusual combined effect of calming violent opponents of a lower power for 60 seconds- and trapping their souls in your Soul Gems if they should die during that time. Sadly, the summoned draugr leave no corpses or goodies behind.
Scouring the room's shelves and burial urns will net you various salables, useful trinkets, and Rjorn's Drum, which can be returned to Rjorn at the Bard's College in Solitude in exchange for training in the six combat skills- Smithing, Block, One-handed, Heavy Armor, Two-handed, and Archery. There's also a rather thoroughly full treasure chest here, which is where you'll find Runil's Journal, if you were sent here to find that, or any other item that you were sent to this dungeon to look for.
Once you're done, if you want a fast way out you can open the trap door in the center of the room. This leads you into a drop to the cairn in the first cave of the system, though there are several partial platforms in the hole that you can use to keep yourself from getting hurt as you fall.
