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This will be a guide on how to build your character to be the most effective. This is from my play experience and style so this may not be the way you want to play. You can use it to judge how effective certain skills are.
The Alchemist is the Mage class in Torchlight. He has good ranged magic and minions(pets). He can fight in close with melee weapons, but health potions become a necessity.
I have foun that some skills are really good while others are severely lacking. There are three skill trees, Arcane, Lore, and Battle. The Arcane tree focuses on offensive magic spells. The Lore tree focuses on minions. The Battle tree focuses on melee combat. For now I will stay out of the Battle tree as I have not spent any time in it. I prefer my Mage to stay at a distance and pick off stragglers the minions missed. Also the Battle tree is the hardest to manage and plan out. So, I will go into an Arcane and Lore hybrid.
The first spells available as an Alchemist are some of the best in the game when leveled up. The Ember Bolt(Arcane) and the Nether Imp(Lore) will be used throughout the game. Ember Bolt shoots what amounts to an Ember fireball. When leveled it shoots 4 at once and does a good deal of damage. The Nether Imp allows you to summon a Nether Imp from the dead body of an enemy. When leveled you can summon up to 6 Imps to fight at the same time. They do some good damage and even that can be leveled.So, both of these spells should be leveled to 10 ASAP. Since their are level restrictions on the skills it will take some time to get it up to full potential.
The real question is where to go from there. Well a good place to go is to Pet Mastery(Lore) which increases your pet and minion armor and damage for each point put into it. It also decreases the amount of time it takes four your pet to get back from selling items in town. Also useful is the Thorned Minions(Lore) which makes it so when your minions will inflict damage on their attacker. On the Arcane side of things Ice Shock is good in the beginning as it will freeze enemies, but in the later stages it just doesn't cut it. Best to skip that one and concentrate on Ember Bolt. The next level of the tree is where it starts to get interesting. Most points on this level should go into the Alchemical Golem(Lore). He is a minion used in addition to the Imps and he is a heavy hitter. As he is leveled he gains a smash attack that will destroy enemies in mass. Once you have him and the Imps you really can sit back and watch them work. Often they will kill enemies before you even get the chance to target them. With them wreaking havoc you have time to launch a few spells and really check out some of the items and do inventory management. There is a point where that luxury dissipates and you will need to stay in the fight to win. You will find the further you go you have to keep summoning Imps and even the Golem.
The next tree level is where you really start to focus on offensive magic. The Ember Lance becomes available which is one of the really handy and cool spells in the game. It shoots a beam of Ember, a la Cyclops of X-Men fame, from the alchemist's hand. It also stays firing as long as the key is pressed down and there is Mana to power it. Its very effective against some of the elite enemies. This is the only thing really worth it in this level. Otherwise level up any previous skills that are low.
This next level is one that goes all the way across the board. In the Arcane tree both Pyre, which is similar to Ice Shock but with fire, and Advanced Spellcasting, lowers the level requirement on spells and regenerates mana faster, have their uses. I would choose Pyre since it is an AOE(area of effect) spell. In the Lore tree there is Burning Bind which boosts you minion's damage for a period of time. Its good to get through those tough areas, but not something that would be used regularly. Finally one of the two skills from the Battle tree is Ember Phase. It basically teleports you to the selected location. It does damage ot enemies on the way out and in from the phase. Don't invest heavily in it. It is good to get out of tight situations and is not to be used as an offensive spell.
The last level of the skill tree. This is where the home run hitters are. All three trees have something you will want. In the Arcane tree we have Ember Strike which is a huge AOE fire spell. It does very good damage. The only downside is it eats mana for breakfast. Still it is a worthy addition. In the Lore tree is the Beam Golem. This is a must. He is in addition to your other minions and blasts enemies with an electric beam. Finally in the Battle tree you have Ember Sentry. Basically this is yet another minion as it is a crystal that floats around blasting enemies with shots of Ember. When it comes down to it take Beam Golem and Ember Strike if you feel your minions are strong enough, otherwise take Beam Golem and Ember Sentry.
The Torchlight Alchemist Skill Tree
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