The Pure Summoner is a Druid that only puts skill points in the Summoning tree. You can certainly make a viable character by mixing summoning skills with elemental or shape-shifting skills, for example using tough Dire Wolves to keep monsters at bay and blasting them with Volcano. However, the build that I'm writing about here spends ALL of his skill points on summoning skills, without exception. This gives you the opportunity to increase your bear and wolf skills to high levels, making them very tough and more than a match for the evil creatures infesting the world of Sanctuary.
As a Pure Summoner, the Druid does not directly damage his opponents. Instead, he uses his "pets" (wolves, bears, ravens) to kill enemies. It's a passive play style, and can work to your advantage in high-lag multiplayer situations because your pets are controlled by the server that's running the game. When you are frozen in a lag spike or stair trap, your pets are active in the game and ready for action. This might make a summoner a good choice for a hardcore play. This passive playstyle is also slow-paced, as it can take some time to wear down the tougher monsters with your minions, so it won't appeal to everyone.
Your playing time is spent repositioning yourself relative to the monsters and keeping your pets alive. You could use a bow or throwing potions if you want to do some direct damage, but you just don't need to when your pets are this strong. The playstyle is similar to a summoning Necromancer, with the Druid having fewer minions and also the big advantage of never needing a corpse. You can resummon your pets as often as you want by just spending mana.
This guide tells how I built my summoner, Caspet, who is level 68 as I write this. It's not the only way and might not be the best way for you. I'm sure there are other paths to follow, for instance - if you don't like ravens you could keep that skill at one point and spend those points elsewhere.
STRENGTH: Just raise it high enough to use your equipment. I went up to 75 so I could use the Sigon's Guard set shield.
DEXTERITY: I did not increase this stat at all, leaving it at the base value of 20. By the time you start needing more Attack Rating to hit, your pets will be attacking for you. I'm using a unique club that gives me +2 to all skills, and it doesn't require any dex to use. The only reason you'd want to increase dexterity is to use a specific weapon, like Wizardspike. Or for more shield blocking, which I decided not to do because I avoid melee combat.
VITALITY: Caspet is going to put all of the rest of his stat points here. Vitality is now 210, which gives him 575 life. I don't use shifting skills that boost life, or Oak Sage except for certain situations, so it's good to raise life with this stat. Also I don't really need stat points anywhere else because I have plenty of mana and enough strength & dexterity to use my items.
ENERGY: My original plan was to eventually get 400 mana, without items. That would be enough to summon 10 bears or 20 wolves in a row. I ended up with 110 energy, which makes 334 mana with items. As it turns out, even when I have to chain-cast my minions in heavy combat, I rarely go below half my mana pool. If I have to re-summon my three Dire Wolves a couple times in the course of a battle, only about 10-20% is used, which is quickly regenerated or refilled with a blue potion. This might be because I normally go into a battle with all minions in place and a full mana ball. Looking back I probably could have kept energy lower (maybe 70-80) and put more into vitality.
Spirits are floating "minions" that generate an "aura" that benefits you, your other minions, your mercenary, and any party members you might have (and their minions too). Only one spirit can be active at a time. Spirits have life points and can be killed by monsters. When that happens, just make a new one. I decided to put one point in all the spirits and try them all out before investing further.
Oak Sage increases the life points of your party. It doesn't just raise the maximum like the Barbarian skill Battle Orders - when cast it instantly boosts current life as well. The main use of this spirit is to make your minions more durable when using them to "tank" for other players instead of doing damage.
Heart of Wolverine increases the attack rating and damage of your party. I decided to add skill points this spirit after testing them all because:
Spirit of Barbs is like the Paladin's Thorns aura. When this spirit is active, and any party member is hit by a monster, the physical damage is reflected back on the monster. Another good spirit for tanking purposes. I like to use this once in a while for fun, just because nobody else does. Plus if you are partied with two other druids, they will most likely be using OS and HoW (and no two druids in a party should be using the same spirits - they don't stack). With skill adders, my single point investment gives me 100% returned. Not of much use in hell mode, where a monster might have 3000 life and do 100 damage per hit. It would have to hit at least 30 times to kill itself on your minions.
NOTE! Druid Spirits are bugged in version 1.09 of the game. The bug prevents you from getting bonuses past skill level 20. So if you raise the skill to 20 and have +9 skill items, you'll still only have a skill level 20 spirit, not level 29. If you plan on having skill items, don't max your spirits.
I like ravens. I realize that almost everyone considers them a waste of skill points. You could go without them, but staying within the summoning tree means you'll have plenty of skill points at higher levels, so it doesn't hurt raise this skill. I pumped it up to level 10 early on and then maxed it after my other minion skills were high enough. If a Pure Summoner doesn't use this skill, who will?
At level 26, you get 5 ravens, who hit 37 times each, for an average of 28 damage per hit. That's a total of 5180 damage for 30 mana! It sounds good on paper, but realize that it doesn't happen all at once. The ravens fly around in circles and zoom in to peck an enemy once in a while. It's more of an area-of-effect damage field that works to supplement your minion's damage. There's no need to wait for a raven to expire before casting another one. They only cost 6 mana each, so I always refresh them by casting five new ones between battles.
Although the damage is low, ravens do have certain advantages. They auto-target enemies and they always hit. Monsters do not recognize them as minions, so they can't be attacked or killed. They can fly over any obstacle. The Holy Freeze aura does not slow them down. They look very cool, IMHO.
Wolves and bears are your central method for damaging monsters. You can only have one type of minion active at a time. So for instance if you have three Dire Wolves and you summon a Grizzly, your wolves will vanish. The three minion skills interact in an interesting way. Each one adds a passive bonus to the other minions. Spirit Wolf is like dexterity - it adds attack rating and defense to all your wolves and bears. Dire Wolf is like vitality - it adds life to all your wolves and bears. Summon Grizzly is like strength - it adds damage to all your wolves and bear's attacks. Even if you plan to use wolves most of the time, you'll still need to spend lots of points in Grizzly to increase the damage your wolves can do.
I was lucky enough to have a druid pelt with +3 Spirit Wolf on it. I used this along with 1 skill point to get through the early stages. If you can't find a helm like that, put a few points here, up to 5. I did not pump this skill for the passive bonus because my pets seem to hit often enough and don't need the extra attack rating (especially with my high-level Heart of Wolverine). They also don't need the extra defense rating - my pets seem to die most often from elemental damage and poison, not physical attacks. Plus when I use Spirit of Barbs, I want them to get hit so the monster's damage will be reflected back.
This was the first skill I maxed, followed closely by Summon Grizzly. You get a maximum of three wolves, who run around with you and attack any hostiles that get close to you. After they kill the monsters, they chew on the corpses and become enraged, doing even more damage to the next monster.
Dire Wolves are my standard pet for normal questing. Having three of them is essential when facing crowds of monsters, to avoid getting attacked. You "control" them by moving around and repositioning yourself relative to the wolves and the monsters. A summoned wolf appears where you click the mouse pointer, so if a monster breaks free of the pack and rushes me, I can resummon a wolf in front of it. If I want to attack monsters across a river or on a separate platform in the Arcane Sanctuary, I summon a wolf or two over there. If a wolf has been poisoned or cursed, I just summon another one with full health. Do not hesitate to resummon a wolf for any reason - they don't mind!
At level 29, each Dire Wolf has 490 life in hell mode (369 in nm, 245 in norm) and does 325-339 base damage per hit.
Use your Grizzly bear when you want one super minion instead of three strong ones. I always use the bear against bosses or when facing up to three strong monsters. My bear is a great tank and is tough enough to fight cows in hell mode. The tactics are similar to using Dire Wolves - resummon the bear any time he's hurt, poisoned, cursed, or in the wrong place. Druid minions don't have any type of Summon Resist like the Necromancers do, so bears and wolves do take heavy damage from elemental attacks. To counter this, just be aware of their condition and resummon them as necessary. You may have to summon quite a few bears while fighting Diablo, while you run around trying to stay out of his lightning-fire attack.
At level 29, my Grizzly has 1225 life in hell mode (920 in nm, 610 in norm) and does 1272-1351 damage per hit. Fight my bear! Fight my bear!
Like other types of summons, you can only have one vine active at a time. They use a lot of graphics to display and can cause lag in multiplayer games with people that use slower computers. I usually don't use vines unless I'm soloing.
Poison Creeper: Crawls around in the ground and poisons monsters. Very weak and does pitiful damage (something like 80 at skill level 20). The only usefulness this has might be to prevent monsters from regenerating health.
Carrion Vine: Eats corpses and gives you life. I put a couple of points here, because it gets rid of corpses that might be revived or used in a corpse explosion. I get 8% health back per corpse, which is 8% of my life, not the dead monster's life. If I party up with a Necromancer who wants to use the corpses, I summon a Poison Creeper - which doesn't use corpses and makes the Carrion Vine vanish.
Solar Creeper: Eats corpses and give you mana. I never put even one point in this because I have plenty of mana and just don't need it.
All of a summoner's skills do physical damage - except for Poison Creeper, which doesn't do enough damage to kill even when maxed. So you need to think about how to deal with monsters that are immune to physical.
My solution to this was to hire an Act 1 Fire Arrow Rogue mercenary. My original thinking was that I could give her a 6-Ort rune bow to do lots of lighting damage, and switch to a fire bow when in areas with lots of lightning immunes. However, my pets are up to the task of dealing with anything except for physical immunes, so the rogue's purpose is now to kill physical immunes, and to provide cover fire for my pets against non-fire immunes.
I chose a Fire Arrow rogue and gave her a fast, high-damage bow that has +1 amazon skills. This means that every arrow she shoots is a fire arrow. Fire Arrows are magic, so they always hit. The Amazon's Fire Arrow skill has a hidden benefit - it convert's the bow's physical damage to fire damage. My merc can take down the physical immune monsters in a multiplayer hell mode bloody hills run. It takes lots of arrows though, so I'm going to get a higher damage bow and a +1 skills helmet for her.
A Pure Summoner doesn't have specific equipment requirements, because he's a focal point for his pets and doesn't directly use items against monsters. The only thing you really need are items that add to skills, because once you have 20 points in your main skills, items are the only way to increase your pet's life & damage. Caspet has a +2 druid club, +2 summoning skills circlet, +3 summoning skills amulet, +1 from Sigon's Guard, and +1 from his Spirit Shroud armor.
Resistances are always good to have (although mine are all negative in hell mode). Boots with faster walk/run mods are very handy because you do tend to run around a lot when fighting act bosses.