Well Necro is made from bones in Skeletron's dungeon.
When I joined their server with copper, fireless gave me the next best sword after silver, which is alright, but pretty bad still.
We, fireless reicon and I, all worked for about 10-12 hours yesterday gaining iron, silver, killing cthulu and eater of worlds by using the 10 lens to summon them which we got the demon and shadow stuff from.
Eater of worlds is harder in my opinion because he is bigger and harder to dodge as well as unpredictable, while cthulu is a big eye that is quite easy to predict and to dodge. Eater of worlds is also the only one that drops shadow scale ore as well as the demonhide bars or w/e. I can solo eater of worlds now without dying, but all of us were dying in a party of 3, even with kryo we were a party of 4 and died at least 3 times each.
Once we all got settled with full shadow mail stuff and full shadow equipment, we did some potion making and killed skeletron, which only took us one try because we took out the hands first, and then the head. The hands are annoying cause they fly around everywhere but when you get at least one down, it gets easier. When you get the second down, you win the battle because his head is so easy to dodge and it's just basically dps from there on out.
After skeletron which of course didn't drop hardly anything, we ventured into the dungeon, got annihilated by anger corpses and flying eyeball thingies and annoying mages, but finally after a while started killing them for bones, and all of us made full Necro armor.
After that, we were able to resist the monsters down there a bit better, got more hp (I have like 300 right now), and explored throughout the entire dungeon.
After that all happened I pretty much just left for the night, came back on today and found that my single player world got messed up. I made a new one, went to corruption to summon an eater of worlds, and on the way to my 3rd orb I was noticing odd textures and ivy's were different. I break open a few blocks to gain entrance, and sitting right in front of me in the Jungle, possibly one of the rarest places in the game. I shit my pants and went in, it is pretty much as hard as the dungeon but there are only 2 monsters I have fought so far (bee and man-eater). It's easier cause there are less monsters, but it's harder to move around because it has those prickles like the corruption. Apparently you can find Cobalt armor there (mage armor) as well as an Ivy Whip which Reicon found. He found the jungle like 10 minutes before I did in Firelesse's server
Just keep working and you will get there. I also suggest digging a hole far into the ground and you will find some iron and other stuff soon.