I'll lend my thoughts on a caster alchemist.
First off, I initially played Torchlight as a Destroyer as getting into the thick of the fighting is the most appealing to me. So I have played with a number of Destroyer builds and some of my base thoughts about a build has come from it.
I have now completed Torchlight on VH HC with a Alchemist caster. It was much harder than a Destroyer build because of the lack of strong armour, and the lack of Frostshield. So here we go.
My platform is XBox360, and you start with a Respec potion and the spell iluvbees. In my builds, the use of a respec potion is always part of the equation. If you don't have access to respecing easily, much of what I say would have to be heavily tweaked. Also, I do not know many of the game mechanics. I have many questions, but don't have a good resource for answers. I'm sure the answers are somewhere in the forums, but not compiled.
I died 3 times on HC before beating it. Each time I died it was because I played too riskily/sloppily. I was just used to Destroyer builds that could take a pounding and got myself into situations that I couldn't spam potions out of.
Alchemist VH Build
The start of a build should begin with a vision I think. I wanted to play a traditional wizard, and this seemed the route to take.
After playing a number of Destroyer builds, my thinking was around offense, offense, offense. The reasons for this was because the more offence you have, the faster you can kill monsters. The faster you can kill monsters, the faster you gain XP, gold, and better equipment. With a defensive build, it takes so much more time to go through monsters, and I really don't think an Alchemist is cut out to be a slugger. Not a traditional caster anyways. So the key to the build is Web/Frost, and high damage output. And running away. I mean, strategic retreat.
I can't stress that enough about my builds; going half-way doesn't cut it. It seemed as I played that if I went a middle route, I would do horribly. So if you're lacking in skill points (and who isn't?) focus on ONE offensive spell, and Critical Strikes. Respect to switch spells.
I also never worried about using potions. If I thought I might need a potion, I used it. If mobs were coming towards me and I was at full health but about to be jumped, I downed a potion as I was running. The cost of potions isn't insignificant, but it's much, much cheaper than buying gear. For the cost of a single piece of gear around the 2000 range, you can buy 20-30 potions, depending. By the time you go through those potions, you might find a piece of gear almost as good or better. So spam those potions! Also, even if you're not playing on HC, death can be expensive. Either in time, money or both.
I would almost never buy gear. At the start, I bought non-magical gear as it was very cheap. I grabbed a high DPS staff with a socket, and threw a poison gem in there to help with Ember Bolt.
I dumped my stats like so:
Magic 3, Defense 2. No exceptions. I used gems I created to add to stat points to make sure I could use the gear I needed. Magic is Key. My destroyers weren't nearly so skill reliant, and magic really increases damage output. 100 magic stat w. Ember Bolt = +50% damage. With LIghning, = +100% damage. Very effective.
Need to have
Level this up every chance you get until you can switch to Ember Lance or Lightning. Even when you have the option for Ember Lance/Lightning, Ember Bolt is still really good with a fast cast speed and a knockback. With a bunch of +% casting gear, I could cast fast enough to just stand still and knock Trolls back so they could never hit me. It gets a 50% boost based on magic score (eg. if your Magic score is 20, it adds +10% damage) and if you slot your weapon with a gem for + poison damage, it combines. So early I bought a non-magic staff with a slot, and slotted it with a Discoloured Green Gem and it really brought my damage level up. High Skill level, +good gems, +high magic score = high damage output and lets you spam this spell dropping enemies like flies.
I leveled this up a lot as well. Ember Bolt takes priority though. It works with all your Damage skills.
Another Great damage dealing spell. This is good for spread out enemies as it auto hits for you, or enemies with a high protection against poison. I found it wasn't as hampered with terrain as well, so found it more accurate. It was also easier on the mana bar. All those pros being said, Ember Bolt leveled up could usually dish out more damage, faster (with the right gear).
Awesome spell. Level this up to maximum. I would heavily rely upon this, luring enemies to choke points and then unloading. The only thing that bolsters this spell I think is +% fire damage. Best against low powered groups, but still can work against bosses. You just sit still when you're using it so against enemies that I'm trying to maneuver, I prefer lightning.
Good when it's available. The damage doesn't scale well though; there's no way to add damage to it, so later on it loses it's shine. But right away it's nice especially if you have Web as well. Eventually I would get rid of it through a respec though.
I haven't done enough testing on this to see whether it's worth it. It's good when you get it though.
Best thing is it lowers the requirements. I put 2 points into this when I didn't have points for anything else. Later on I'd add to it. It is definitely worth it to put in a couple points here early IMO; being able to wear stronger armour helps so much.
Something you invest in endgame.
Nice to have
I put only a single point in here. The only thing worth having is the mana regeneration, and I find it's nice to not burn through many potions. However, as enemies generally come in waves, for difficult fights you'll still need to spam potions. The mana regeneration just doesn't cut it. So as the big fights are more concerning to me, I've only ever spent a single point. Another 'nice to have' skill.
I think this skill isn't as good as people make it out to be. Think of it like this: if you can go through monsters 2% faster (by increasing damage output), then that's just as good a 2% experience/fame gain. I only level this when I have nothing else to get.
Barter/Treasure Hunter: no real explanations needed here.
Ember Phase. I did not use this on the 360 as it didn't have a point and click mechanism for fine control. On the PC it might work a lot better.
Thorned Minions: This may help against bosses combined with a pet that heals itself. As it's a reflection of an opponents damage, it might work really well. Haven't tried it yet, but want to.
I haven't had lots of extra skill slots, but if I do I think this will come in handy. With Web/Frost I assume it would freeze them longer, Elemental Overload would last longer/do more damage, and Dervish you would greatly increase your attack speed. At high, high levels this is probably very important.
Good spells to get are:
Web or Frost
Elemental Resistance (key in the end game)
Web and Frost are really good and you should have one of them. Used correctly at choke points, it can allow you enough time to unload magic to take out enemies before they get to you. Highly valued, although Web is the best but much more rare.
Heal Self or Heal All
Some kind of summon
I just relied upon potions for myself; your pet can't heal you fast enough. I need to look at the cool downs/mana cost, but if Heal Self is cheaper (and it should be) I'd rather have that on my pet early, and then Heal All when he gets a good summon. I see people recommending crowd control; web is so rare I haven't had a chance to try it on my pet, but my pet makes such dumb decisions and these are such tactical spells I definitely wouldn't rely on it. I'd use web/frost on myself first, and if I had spares send them to the pet to try.
Increasing damage output decreases the need for protection and decreases potion consumption (both red and blue). It also busts through an opponents resistances. Therefore, I focused on high damage output, armour, and resistances. As most spells damage output is the spell itself, faster casting is EXTREMELY helpful. It also helps with combos (Elemental Overload, Web, spam attack spells).
Usually I would go with a high damage Staff slotted with the best offensive gems I could find. I would try and stack damage (if my main spell was Ember Lance I'd stack fire damage on it). The reasoning for me is if an enemy has a variety of resistances, it's better to lay it on thick with one damage type and 'punch through'. I do not know if this is true or not; haven't tested it. Just a logical assumption. The staff is also good for weak mobs that make it through your spells.
Adding gear that adds HP is important as well so you don't get 1-shotted. A Destroyer hsa plenty of hit points so reducing the damage each attack will do is key, but I've had better luck just loading up on +HP gear. My current build is around lvl 20 and has over +500 HP in gear (looking for more as well). I think this will help protect you against most attacks better than resistances.
Pet Gear: My pet is to distract long enough for magic to do its work. I stack it with +Magic Find, +Defense, and +HP in that order. I try to make sure I have a couple of fishes to feed it even though I hate fishing. It can be invaluable. Finding good magic items is the best way to increase your effectiveness so that's my first priority with my pet.
Make sure you have enough potions, and don't cheap out on them. You normally use potions when you're low and in a big fight, so get the good stuff. It can really save your bacon.
I almost never buy gear. I would rather invest in potions. The only time I do is if a piece of gear is really lacking and a buy greatly increases its effectiveness. Buying a spell is OK, but I wouldn't do it to merely increase it a level. Elemental Overload 2 works just as well as 3 pretty much. Same with other spells. If it's a jump of 2 or more, I think its worth it, or if I just have money to blow.
I never sell magic gear; I always transmute it into Gems. Having a good stockpile of gems allows me to tweak my stats to wear the next best gear available, and stack elemental damage.
Armour I focused on Elemental Resistance and Armour. Both are important. Faster cast is also really good because it translates into damage.
My money went into potions. All magic gear I didn't want I transformed it into gems and socketed it. If a particular piece of gear was lacking, (if I could find a decent pair of gloves for awhile for example) I would buy one. I chugged a ton of potions but it didn't bother me.
Depends what spell you're using, but the most dangerous part of a dungeon is the first little area. Once you go in, you can always retreat, especially if you have web/frost. If you're in a pinch and looks like you'll be overwhelmed, use a town portal to escape, then do some other dungeon.
WIth a high focus on Offense, this build is riskier. However, if I didn't feel comfortable descending in levels, I would buy a Map and clear that level, or go to a higher level in the dungeon. Remember that higher offense means you can clear enemies faster, and get magic items/gold/XP/fame faster as well. Thats why I think it's a stronger build than a defensive one. If you go defense, it may be stronger at a particular point in time (may; haven't tested) but you won't improve your guy faster. Once he's improved you can always switch to defense.
As long winded as this is, I have not taken any builds past level 35. I do not know how they scale from 35-100; once the main quest is done it feels too much like a grind. Once I've experimented with all the classes I'll continue past level 35 and see if I can make it to the end of the Shadow Vault. My strategy may change from that experience.