1) Dealing with Dreamland bosses with the poison necromancer.
.I think the following could apply for "ranged" builds but such certainly goes with my experience as a poison necromancer. I'd say that such a build is in general the most effective since getting most of such equipment s fairly easy and not that expensive and yet has a lot of bang for its buck. The only hard things you'll have some trouble getting are say carrion rings or SOJs for you necromancer and the unique dagger that the Lich King drops. I mention this to give a general idea of what equipment you ought to have to get you by given the circumstances (you should certainly make sure you have belials or the cow queen set before doing the following). I could elaborate a bit further on this matter, but I'd like to jump into some tactics first.
First Kaalut. He's actually a joke if you can get him "trapped". Quotes are needed here because sometimes he just phases/walks over the stone boundary around the cathedral/cthulu lair (don't ask why it just happens though it tends to only happen when you get a bit too close). All you pretty much have to do with him is lure him to the corner opposite where you should be (may post a screen shot if necessary). There must be a wall between you two, and you ought to try to keep him barely within screen distance and simply spam maelstrom until he's dead. If you are about that distance away from him as I suggested, then you shouldn't get hurt at all by him (unless as stated he just moves right through the boundary). At around 200k dmg, this takes me 2 to 3 minutes. Not so bad I suppose considering my gear.
Next up is the snake lady. She's probably the easiest to deal with for pretty much the same reasons except now she has some minions around her, and you know what that means: corpse explosion. You want to do a similar thing that you did with kaalut but use the small stream as your "wall". Keep the enemies as far from your sight as possible. If you must get closer for whatever reason, spam maelstrom a few times and sort of strate left and right in case the boss throws some blades at your direction. Time to kill: maybe 1 to at most 3 minutes.
Mappo is where things get a bit harder especially given the area and the speed of the boss. I find that it is better to simply try to lure the boss more out in the open i.e. closer to the portal that brings you to the dreamlands. There you'll have to do a bit of a tango with mappo. Spam golems a bit, use maelstrom, and move. Spawn more golems, spit more poison, and move. Rinse and repeat. It's not a particularly appealing method, but it works. Estimated time to kill him. Maybe around 5 min (I'll do another run shortly to see).
The eyeball boss is where things get annoying fast (as of my build now anyway). He will likely take you the longest to kill. I personally would recommend avoiding to have to fight him with a poison necro unless you feel you have no choice. Better to get a friend or clan member with a zeal paladin to deal with him while you deal with the rest.