Magicians are the Masters of Invocations, they can invoke the Strongest pets in the game, multiple gears and accessories and have a wide range of utilities.
Far from being only limited to this role, Magician are excellent damage dealers and follow closely Wizard in the capability of sending your enemies to the ground.
Take time to read the Getting Started as it'll give you an easy way to level using your pet.
Solo: Magician is a class well suited for soloing. You have access to the best pets in game, you can decently heal them and add your nukes to fasten the process. As most of the Pet owner you can easily level solo to 65, with Lazarus modifications to pet aggro, you can even add some extra DPS using a good proc melee weapon.
Melee Group: As a DPS class, and relying on your pet you can easily plug your Magician in a melee group, melee group usually having Shaman instead of a Druid, you'll miss the +35% spell dmg aura, but with your pet this won't impact too much your DPS. You also have access to the Call of The Hero Spell allowing you to summon members of your group to you, this is really useful for melee group who usually lack invis and easy way to travel. And don't forget your Damage Shield it add to the group overall DPS.
Caster Group: Magicians fit well in caster groups, particularly groups with multiple pets as you will be there to summon new pet equipment if a pet dies or an enchanter DC a mob. You can even swap your tank for a Magician pet, it won't be as efficient, but for leveling purposes it can work well. Your direct damage are close to wizards ones even if innate critical chances of Wizards will make them higher spell dps. With Force of Elements AA, your single target spell dps during xp grinding will be higher than a Wizard's (but not close for raid bosses). You also have the capability of summoning mod rods, they allow those who use them to convert hp to mana, allowing your group to regen their mana faster. With the Large Modulation Shard AA, this can be a significant amount of mana, rivaling Necro Mind Flay and Enchanter Second Spire but not as much as Beastlord Focused Paragon.
AoE Group You gain access to some strong AE spells so you can replace a Wizard in an AoE group, you won't be as fast as them, but you can easily do the job.
As a DPS class, your main role is to do damage to your enemies, you have 3 main ways to do this.
The first one is your summoned Pet, you need to know them and know when to use them, note that after lvl 50 and the Vocarate spells line each pet stop being warrior only and start to have their specific class.
Earth Elemental are the tanks pets, they have a short duration Root Proc and the highest HP/AC, they stay warrior from start to end.
Water Elemental are the best melee DPS pet, they proc a DD spell and becomes Rogue at 54, and start backstabbing. They have decent HP.
Air Elemental are melee DPS pet, but they proc a short duration stun, making them really useful against casters opponent as stun can interrupt their castings, they become monks at 53 and so access to the mend ability.
Fire Elemental the highest DPS pet, but the weakest HP wise. They have a Damage shield and a DD proc and become wizards at lvl 52 so start chain casting really strong DD (2660 DD for Child of Ro at lv 63). This is the highest DPS pet until you get your epic pet, and in some situations will out damage the epic pet because of the burst dps.
Epic Pet is boosted here on Lazarus, and really strong, with a 66 Damage Shield and a 6s stun/200 DD proc. An epic pet with increase pet power 30 and invested AAs will do 700 dps on its own.
Don't forget to set your pet buffs, especially the Burnout line. And your pet Heal, it isn't as efficient as healers heal but it can save them some mana, and save your pet.
An easy trick with late Fire Pet is to set it as a fire turret by buffing him with the kindle Spell, it will give him a 5000% (base mana) absorb shield, a 12 mana regen, a 10 dmg shield and root him, allowing him to chain cast his bolt and take serious damage without problem. Just remember to click it off when you'll need to move to an other spot.
Your second way to deal damage is your spells. You get really good fire and magic DD, AE and PBAE spells. You also have access to the VS elemental line of spell, check Using Ifs to set an Ifs to allow them to automate cast only on elementals.
Your third way to deal damage is with swarm pets. Your swarm AA Host of the Elements can do massive amounts of damage on it's own. It has 12 ranks and they're expensive, but once you get them it will do 1600-2200 dps to raid targets on a 10 minute cooldown.
With these 3 ways of doing damage (and a bunch of AAs and focus effects), Mages can rival Wizards for top DPS.
Last function you get is buffing and feeding your group with Summoned items. Always put a Damage Shield on your tank, or the characters who often takes hit, don't forget to feed your mana users with Rods of Modulation Gimme Supply function is really helpful for this, and set also pet weapon function in your ini, it'll help others and will allow you to equip your crew pets easily.
The standards spells in pet weapons are Muzzle of Mardu (11% haste) "Grant Spectral Plate" (300 AC/480HP and other stats), Summon Jewelry Bag (Focus items), Blade of the Kedge and Fist of Ixiblat (Pet weapons)
With Gimme Supply you can pre-summon all but the jewelry bag items and the first time you need to a equip a pet it will be near instant. It can also be helpful to pre-summon Mod Rods and Large Modulation Shards.
[Magician] Auto-Pet Weapons (On/Off)=Off Summoned Pet Item=Muzzle of Mardu Summoned Pet Item=Grant Spectral Plate Summoned Pet Item=Summon Jewelry Bag Summoned Pet Item=Blade of Walnan Summoned Pet Item=Blade of the Kedge
[Supply] Supply=Summoned: Muzzle of Mardu|Muzzle of Mardu|10s Supply=Summoned: Blade of Walnan|Blade of Walnan|10s Supply=Summoned: Blade of the Kedge|Blade of the Kedge|10s Supply=Folded Pack of Spectral Plate|Grant Spectral Plate|10s Supply=Summoned: Large Modulation Shard|Large Modulation Shard|10s Supply=Rod of Mystical Transvergence|Magi`Kot's Pants of Convergence|10s
- Pet Affinity - so group buffs and heals will hit your pet
- Force of Elements - large and free instant dd on a 30 second cooldown, can crit for 20K
- Quick Damage - reduce damage cast time
- Pet Flurry and Crit - pet dps costs you nothing and increases the speed at which you can grind XP
- Modulation Shard - Large Modulation Shard converts 4000 HP to 3000 mana, as you get your healers up in AA it becomes very cheap to get the HP back from this conversion and so it's a near-free 3000 mana every 5 minutes, which greatly reduces downtime
- All the % crit increase and crit damage increase AAs
As most Casters try to find heavy focus gear. Your best focus will be pet Focus, try to find the highest possible so your pet will be stronger, second best focus are Mana cost reduction, damage augmentation and casting time reduction.
Elemental silk armor is a good bet as it got a lot of focus and decent stats.