Skill Trees

You will obviously pick the Hydra and all its prerequisites. You should also max out the synergies to greatly boost your Damage. At this point, you will notice the Lesser Hydra has similar synergies, but gains twice the bonuses, which is why it will be slightly stronger during the campaign against Bosses. 

The Fire Mastery simply improves your Fire Damage, but remember that Fire Pierce does not affect your Hydras. The first level provides insane value, while each next level is only 4% increased Damage. Dump all your free points here once you have nothing else to max out. 

Optionally, you can invest in Meteor to deal with Fire Immune Enemies during the early progression, but it is not really worth it in the long term. Later on, you should either break their Immunities with certain Items or skip them for the sake of efficiency. 

Your mobility comes from two Skills: Blaze and Teleport. Leave them at level 1 just to unlock their functionality. In the Lightning Tree, you will also want to fully invest in Energy Shield. This Skill is not worth using if you can't commit all points there, as each hard level drastically improves its effectiveness. In Project Diablo 2, it no longer scales with Telekinesis. 

The Shiver Armour provides a decent survivability boost for a mere investment of 2 Skill Points. It improves your Defense and freezes your Attackers. 

Gear

It is a fairly standard Spell-caster Build, although you don't need to focus on Faster Cast Rate at all. You will only need bonuses to Skill Levels, Fire Damage, and lowering Enemy Fire Resistance. The defensive aspect is fairly unique, as you will rely on Energy Shield, which means any bonuses to your Mana are very valuable. You can ignore Elemental Resistances, as they do not affect the Damage dealt to your Mana. Although Poison bypasses Energy Shield, you can simply drink Antidote Potion instead. A bit of extra Life and Resistances are still useful to have to avoid becoming defenseless once you run out of Mana. 

You should aim to use Eschuta's Temper or the new Embersworn paired with Phoenix Shield. This Runeword Shield greatly lowers enemy Fire Resistance and provides amazing sustain through Redemption Aura. You can also use Dragon Shield instead for a massive boost to your Damage. This Runeword should be used on your Body Armour instead, as there are not that many useful options otherwise. If you lack defense, you should boost your Energy Shield with the Memory Runeword, which grants 6 Levels for that Skill. In your Weapon swap, you will be running the standard Call to Arms and Lidless Wall combo to get beneficial Warcries. 

There are no better Gloves to wear other than Magefist. Similarly, anything other than Arachnid Mesh as your Belt is suboptimal. For Rings, the Stone of Jordan is ideal due to Skill level and hefty amount of Mana, although you can also opt for Wisp Projector, Kadala's Heirloom, or Bul-Kathos' Wedding Band if you happen to find them first. The Raven Frost can be used if you lack protection against Freeze. 

The most common choice for Amulet is the Mara’s Kaleidoscope, although you might also get a cheaper Tal Rasha's Adjudication for +2 Skills, or a Magic Amulet with +3 Skills and something useful on top of that. 

For the Helmet, you should ideally strive for the Shako, but if that's still unobtainable, you might choose the Kira's Guardian instead. 

The Boots offer some flexibility. The Silkweave offers a decent boost to your Mana, although the Merman's Sprocket offers better sustain and much higher Movement Speed. The War Traveler is also a good option if you lack Magic Find. 

Charms

The Charms are very simple and straightforward, as they follow the logic of your previous items. The most important Charms are Fire Skillers, ideally with something extra on top of that. 

The Small Charms are used to fill the gaps: you will want Life, Resistances, Faster Hit Recovery, or Magic Find there. 

You will obviously want all 3 Unique Charms: Gheed's Fortune, Hellfire Torch, and Annihilus. All provide very valuable bonuses, which reflect on their price.