I actually just rolled up a fishymancer after a two year hiatus from D2, and I'm a bit disappointed so far. I don't remember a lot of the technical aspects of the game and I don't have access to really good gear, and because of that I feel my killing power is diminished.
I just reached level 25 and I had quite a difficult time with Mephisto. I've maxed out RS and I'm working on SM right now, but my skeletons are too weak. They get killed quickly by Mephisto and the council members, and there aren't enough corpses to revive all of them.
Of course, my gear is likely a problem but I don't feel like I have much of a choice until I reach NM or at least Baal.
The only way I feel like I'm able to progress effectively is with my friend who recently rolled up a Kodiak Druid and can help tank with my skeletons. Unfortunately, that means soloing isn't very fun.
If you need a better wand, there are several options for cheap:
1. Find a Bone Wand with +3 Raise Skeleton (and possibly +1-3 Skeleton Mastery). Either socket it with your Larzuk quest or buy one with 2 sockets. Make the rune word "White" (runes Dol + Io) in it. Ask people if they have any spare Dols or Ios, because these are commonly considered cheap or worthless runes. This will give your army a big boost and you'll have nice Corpse Explosion from it, too. ("White" adds +3 to Poison and Bone skills, of which CE benefits from, and +4 to Skeleton Mastery, which again benefits you.)
2. Find or get a 4-socket Crystal or Broad Sword. Make a "Spirit" in this with Tal Thul Ort Amn. If you do not have these runes, trade 1-2 perfect gems for them each. A 4-socket Crystal Sword should also be available for a few pgems or runs in the later acts of Normal or throughout Nightmare.
3. Buy a Summoner's Wand w/ +3 Raise Skeleton. It just costs $$. Later on, buy a +2 all skill wand or +3 Summoning wand.
If you need a better shield, look for the following:
1. +Raise Skeleton or +Skeleton Mastery on a necro head. If you get a 2-socket Necro head with +3 Raise Skeleton, +Skeleton Mastery, or both, socket it with the following rune word: "Splendor" (Eth + Lum). This adds +1 skills, 10% fcr, and some other nice stats, including magic find. Eth is commonly dropped in Normal and a Lum may be gotten from a bad Normal or NM forge (or worse, a Hell one, like I have had several times in the past). Lum may also be obtainable for a perfect gem or two.
2. "Rhyme" offers good resists, better blocking, and some MF. Shael + Eth. Put it in your favorite 2-socket shield or Necro head.
3. Sigon's Shield. +1 all, better blocking, etc. I'd personally take a Necro head over this one ASAP.
4. "Spirit" Monarch. If you find a 4-socket Monarch or a white Monarch in late NM or Hell, make it into a "Spirit" ASAP. You will love this shield. While it doesn't boost your skellies as much as a good "Splendor" head would, you can spam CE fast, making everything on the screen all gooey.
General tips:
1. Make sure you are casting Amp damage on all enemies. This will (roughly) double your skeletons' damage output. (Actually, it's -100% to enemy PDR, but let's just say it's double damage...)
The last few nights, I've been playing with Summoners who weren't using Amp or Decrepify at all on Bnet. During Baal runs in big games, this is bad. Really bad. Do not make this simple mistake. People might hate your skeletons, but they will not hate you for casting Amp. Even then, tell them to stuff it.
2. Against bosses, use the time-honored tradition of Clay Golem + Decrepify. This combo will slow enemies to a crawl, so your skeletons can beat on them. Cast Clay Golem right next to the offender(s) so he can get in a few hits quickly and slow them. Against Council and Meph, this may mean repeated and fast casts of Clay Golem targeted to whoever needs slowing.
3. Buff up your Merc. The Merc becomes increasingly important a powerful Fishymancer's army. Let's discuss this briefly.
How to make a macho Merc:
1. Might Merc > all other Mercs. Exception might be a Rogue Merc + "Edge" Bow + "Bramble," but just keep in mind that the Might Merc makes your skellies strong, and he deals out a lot of damage on his own.
Later on, this will become more important when you need to get bodies on the ground fast for chaining together lots of CE explosions for fast kills in Hell.
2. Get a good weapon on him, so he can deal a lot of damage. "Insight" (Ral Tir Tal Sol) in an ethereal polearm, whether it's exceptional or elite, will raise his damage a lot and give you a lot of mana to play with. It also comes with some MF on it.
"Honor" in an ethereal exceptional/elite polearm isn't bad, either, because it has high ED, good stats, and +1 all skills. This is made with Amn El Ith Tir Sol. Later on, get a decent unique like Bonehew, one of the unique spears, or an ethereal "Obedience" on him for tremendous damage.
If you have no other option, make a "Strength" Scythe or War Scythe with Amn + Tir, so you can have a weapon that grants 25% Crushing Blow to your Merc.
3. Get lots of Crushing Blow on Merc. This makes your life around bosses and other hard enemies much easier. You can get it from some weapons like Hone Sundan Yari, rune words, armors, and helms like Guillaume's Face.
Example high-damage setup which I use on my Fishy's Might Merc:
Ethereal "Obedience" Thresher (for fast speed and more hits, which equals more CB opportunities)
Andariel's Visage w/ IAS jewel (35% IAS + 2 skills + leech) or Stealskull w/ IAS jewel (25% IAS + leech)
"Treachery" armor (procs Fade, has 45% IAS, makes Merc poke very fast)
He deals 6K damage with the above gear before I cast Amp. Bosses are really easy to kill.
4. Load him up with +skills, especially on a Might Merc. More +skills = higher level of Might = more damage from skellies and Merc. Arioc's Needle (+4 all skills, good damage), +skill helms, +skill armors, etc. These all help.
To help your summons reach their full potential, keep in mind the following:
1. When summoning skeletons, use equipment with the greatest amount of +skills to "prebuff" your little ones. Even if one or two die after a switch, they will retain their summoning level and bonuses. Therefore, you should have a "prebuff" setup and a "normal" exploration setup on your guy. Invest into it. It's worth it.
2. Summon Resist. 1 point in Summon Resist is all you need. If skeletons and golems don't have resists, they die.
3. Golem Mastery. 1 point at least. Faster golem, stronger golem, better golem. If you want an Iron Golem, consider maxing this. Certain Summoners like to keep an "Insight" Iron Golem (a Golem made from something like a Voulge "Insight" for low damage and the aura) so they can give their Merc something other than an "Insight."
Playstyle Evolution:
1. The combo of Amp + skellies will carry you through most of Normal and Nightmare easily.
2. As you start into your 60s or 70s, you might notice your skeletons losing their effectiveness if you don't have many things to boost them. At this point, they function better as mobile bone armor than they do as main damage dealers. Boost your Corpse Explosion or max it to counter this and prepare for Hell. You can choose to boost Amp Damage or leave it at 1 point. I left mine on my Summoner at 1 point but I maxed CE.
3. Now, without enough +skills, you may notice your skeletons kill slowly on their own. You need to change your expectations a bit. Make sure your Merc and skeletons are prepared as best as possible, because they have one important mission: to gather corpses. As soon as the first corpse drops in a fight, you should make sure every enemy is Amped, and then explode that corpse. With high CE, you can start to get multiple overlapping explosions, which create a chain reaction, with more explosions leading to more corpses. In solo and small games, this will be the main way you clear the entire screen in milliseconds.