When you talk to Cain after you’ve been in lev. 14, he’ll tell you about Lazarus the betrayer. Lazarus can be found in lev. 15 in his own chamber in the North Western part of the level. (In SP, you must give the staff of Lazarus to Cain in order to get the quest. Then take the red portal in level 15 to Lazarus' Tomb.)
The most important part about how to handle Lazarus and CO (Blackjade, Redvex, some hellspawn and some advocates) is that you don’t face the whole bunch at once. Since the shape of his chamber differs in SP and MP, different tactics can be applied. I'll start with MP:
Come close enough for Blackjade and Redvex (the two unique witches) to see you, then turn around, and walk away. (If you’re not so strong, luring one out at a time might be better.) They will follow. Kill them on safe ground. You can easily kill them by using stone curse, or (if you’re a warrior:) the peek-a-boo or telekill method (mentioned in the warrior part.) You could also throw some firewalls on them and then start walking parallel to this firewall, just like killing unique poisonspitters (described in the poisonspitter chapter.) After you killed them, go back and lure the hellspawn away. Kill them on safe ground as well.
Now it’s just Lazarus and some advocates left. They cast deadly fireballs, so make sure you’ve got a decent fire resistance if you go back in. There’s another way though. Just send in a golem. Lazarus and the advocates won’t use their fireballs on the golem; they will only use flash, which is a pretty harmless spell. (Got that from Bolty’s page.) Of course the golem can get destroyed, all you got to do then, is make another one, and send him in. This method can take some time, but it works, while you’re not in real danger.
You could also walk in, stone curse Lazarus and some advos, and smash them. Stone curse is a very useful spell, effective against all uniques except Diablo himself. Of course you can always combine these two tactics; stone curse them, and send in a golem.
You could also walk to the entrance of his Lair, cast some firewalls on some advos or Lazarus, and then walk away again. They will burn while you do nothing. Since Lazarus is a tough fella, you might have to repeat this tactic. (Note that Laz has to be activated before you can kill him.)
When you play with a Rogue or Sorcerer, perhaps you can kill some advos before entering the room again. Just pump some arrows/spells (like fireball) into the chamber, when you hear some sounds, you hit something. This way you can take out some more monsters, before entering the chamber again.
When you play MP, and when you don’t have that much trouble with Laz and CO anymore, I suggest you do some Laz runs. “What the heck is a Laz run?” you might wonder. Well, start a game, go to Laz’s chamber ASAP, and kill everything in there. Because there are three unique monsters in there, you’ll get some magical items (or books) when you whack them. And because you can find better items in lev 15 (Laz’s level) than (let’s say) level 13, you could find some really cool stuff. Of course you could also find crap, but there’s always a chance to find something awesome. Anyway, after killing Laz and CO, and picking up the dropped items, start a new game, identify it, sell or keep it, and go down to kill Laz and CO again. This is THE way to find some really good stuff. Good luck!
Now SP:
In SP you get teleported into his Lair. (It is wise to save before taking the teleport in (on the place where you first entered his Lair this teleport can be found.))
Immediately throw some firewalls on him, then run away before he finishes his speech. Run around the corner, and wait (politely ;-) till he stopped talking. Now step around the corner, and immediately step back. This way you activated him, but because he merely saw you a second or so, he won't move. Since he is activated and is standing in some firewalls he'll burn alive.
As for the two unique witches and the hellspawn (which were hidden behind a wall) I'd say use the same tactic with the firewall as I described to kill unique poisonspitters. (Move parallel to the firewall (not increasing your distance to it and dodging their blood stars) while they burn and shoot at you.)
