Elementalist

As the Kobolds surrounded him, blades poised to strike down the Half-Elf, he let out a smirk. Mist crept and slithered around him, forming a twister. With a flick of his wrist, the Half-Elf commanded the Elemental to attack. Soon it was the Kobolds screaming and begging for mercy, not him like they planned.

The light reflected off the polished curved blade of the Hobgoblin, momentarily blinding the Elf. Quickly blinking away the sudden flash, the Elf drew his falchion, the decorative gold flames making the weapon more of an art piece than to be used in combat. Even then, the Hobgoblin takes caution as the Elf's sword is covered in a blaze of red fire.

As the buildings are being raided and burned around the human, she kept approaching the bandits. They turned to her smugly, letting out a few laughs, and chucked a vial of acid at her. Then fear started to take control of them, and the acid was being absorbed into her skin.

Some Fighters are interested in the physical world, and see it to be manipulated to help them and their friends in there endeavors. Those who take the archetype of an Elementalist are those who have studied with druids or wizards, and have been inspired to learn magic about the world and it's workings. 

Elemental Weapon

When you choose this archetype at 3rd level, you can infuse your weapon with an element an amount of times equal to your Intelligence modifier per long rest.  Choose either Fire, Earth, Water, or Air. This applies 1d6 of Fire, Acid, Cold, or Lighting damage to your weapon, and the die increase by 1 at 7th, 10th, 15th, and 18th level, and the weapon also becomes Magical to overcome resistances and immunities. If the weapon is disarmed from you, it loses it's charge and you must use another to replace it.

Spellcasting

You have access to the following spells from the Wizard spell list:

1st level spells - Absorb Elements, Burning Hands, Earth Tremor, Feather Fall, Fog Cloud, Ice Knife, and Witch Bolt
2nd level spells - Aganazzar's Scorcher, Continual Flame, Dust Devil, Earthbind, Flaming Sphere, Gust of Wind, Levitate, Maximilian's Earthen Grasp, Melf's Acid Arrow, Scorching Ray, Shatter, and Snilloc's Snowball Swarm
3rd level spells - Erupting Earth, Fireball, Fly, Gaseous Form, Lightning Bolt, Melf's Minute Meteors, Protection from Energy, Sleet Storm, Thunder Step, Tidal Wave, Wall of Sand, Wall of Water, and Water Breathing
4th level spells - Control Water, Elemental Bane, Fire Shield, Ice Storm, Stone Shape, Stoneskin, Storm Sphere, Vitriolic Sphere, Wall of Fire, and Watery Sphere

You use Intelligence as your spellcasting ability. You learn 3 spells at level 3 and gain an additional spell each level. For notes on spell slots, you are a half caster.

Elemental Attunement

At 3rd level, your study of nature and the physical world has given you better understanding of what is has to offer. You gain proficiency with the Nature skill and Primordial. 

Elemental Resistance

Starting at 7th level, you have learned to protect yourself from the elements. You gain a bonus to spell saves that involve fire, cold, acid, and lightning damage that is equal to your proficiency bonus.

At 10th level, you can ward magics you have studied away from you. After a long rest, you also gain resistance to 1 of the mentioned damages, and you can change it after a long rest as well. You also no longer suffer exhaustion from hot or cold climates.

Absorption

At 10th level, you can cast Absorb Elements as a 3rd level spell at will, without expending a spell slot. If you already have the spell, you can replace it for another.

Elemental Companion

Starting at 15th level, you can preform a 1-hour ritual to summon a CR 5 Elemental, and choose which Elemental it is out of Fire, Earth, Water, and Air. As a part of the ritual, you must use the twice the material components of the Conjure Elemental spell. The Elemental is friendly to you and your allies, has temporary hit points equal to half of your hit point maximum, and is summoned until it dies or you choose to dismiss it.

If the Elemental dies, you can preform the 1-hour ritual to create another.

Elemental Master

At 18th level, you become one with the elements. After you finish a long rest, you can choose to gain immunity to 2 damage types, you're choices being fire, cold, acid, and lightning damage. 

Also, once per day, if you are ever hit by an attack you have immunity to, you can redirect it to the source of the attack and deal 1/3rd (rounded up) of the original damage. The immunity has to be one of the four previously mentioned.