This is a hyperaggressive deck that is all about strong openings. Elementalist's additional threshold allows to have those openings consistently and also allows for many neat game starts which are not possible with other avatars.

Possible Openings

Turn 1

Troubled Town -> Serava Townsfolk
City -> Kettletop Leprechaun/Bosk Troll
Tower -> Kettletop Leprechaun/Bosk Troll
Tower -> Highland Princess/Sir Tom Thumb

Turn 2

Any 2-cost drop
Any Site + Pond -> Autumn Unicorn/Lugbog Cat/Tufted Turtles
Water Site + City/Tower -> Lugbog Cat/Order of the Sacred Oak/Sir Pelleas
Earth Site + City/Tower -> Autumn Unicorn
Any Site + City/Tower -> Tufted Turtles
City + Pond -> Sacred Stag/Gyre Hippogriffs
City + Tower -> Gyre Hippogriffs/King Arthur
2x City -> King Arthur

Turn 3

Any of your 3-cost/4-cost drops
Double 2-cost drops/Serava Townsfolk (Troubled Town)
City + Pond//Tower + X: 2 cost drop + 3 cost drop
City + Any Site + Pond -> Questing Beast
City + Water Site + Tower -> Questing Beast
2 x City + Water Site -> Questing Beast
City + Tower + Floodplain -> Lilith/Lacuna Entity
2 x City + Floodplain -> Lilith/Lacuna Entity

Ideally you want to either have a

Turn 1 play followed by any other Turn 2 play
Turn 2 3-cost drop/4-cost drop followed by another 3cost/4cost drop on turn 3
Turn 2 2-cost drop followed by 3-cost/4-cost drop on Turn 3
Turn 1/Turn 2 Boulevard of Bones combined with any play on Turn 2

And you want to mulligan aggressively for that. Avoid keeping any interactive spells or expensive minions in your opening hand (unless you have 2 early minions already)

As you can see above, the Cities are most important for many of the busted openings. They especially allow you to cheat out minions above curve multiple turns in a row. You usually always want to keep them in your opening hand. Similar is true for Towers + Ponds, but you want to make sure you have the right threshold for your minions.

Midgame

Once you are ahead on the board with your above curve opening, this decks has a lot of tools to stay ahead:

Torshammer trinkets allow to trade favorably
Fey Changeling is a very flexible "answers all" tempo play. Notably it ignores Wards since it is not a "target" ability. It is also a great answer to Infiltrate/Mesmerism.
Grapple Shot/Whirling Blades are always great removal + repositioning + extra damage tool if you are ahead on the board
Infiltrate is a very strong spot removal (IF opponent has no counterplay)
Apply Alkahest is very versatile removal that can destroy minions, auras, artifacts, can destroy burrowed/submerged minions like coy nixie or root spider, can act as a small Mass removal if enemy bunkers multiple minions at one site
Pollimorph is a very mana efficient spot removal
Abaddon Succubus (especially with Wuthering heights) dish out extra damage allowing favorable trades.
Above Curve played Lacuna Entity can eat opposing early drops
Mesmerism is Mesmerism

The additional Thresholds and low curve of the deck means you don't need as many sites: Ideally you want to never draw extra sites at the start of the turn, but always draw spells. This allows you to get ahead on spells early on, similarly like Geomancer/Pathfinder do. Don't panic if you don't have the right threshold for one of your spells in hand, you will eventually draw the right sites naturally with your Elementalist tap ability or Leprechauns. This of course doesn't mean that you NEVER want to not draw an extra site at the start of the turn - sometimes there are situations where you need to do it - but weight on it adequately.

Once you have enough sites in play, you want to move your avatar forward. We play quite some removal that requires the targets to be "nearby" like Apply Alkahest, Pollimorph or Mesmerism. Additionally this deck has a strong focus on Ring of Morrigan:

Torshammer Trinket & Sir Tom Thumb allow you to trigger it repeatedly
Highland Princess can search up the Ring or additional Trinkets
Full set of Toolbox gives you another option to trigger it multiple times easily
The Low curve and the game plan of starting to draw spells early allows you to play out multiple spells in a single turn in the late game
Troubled Town can act as a cheap way for an additional trigger
You can combine Ring of Morrigan with Succubus/Tufted Turtles to get rid of bigger minions

Battling defensive Sites + Minions

Sites like Thin Ice, Troll Bridge, Gnome Hollows or defensive minions like Coy Nixie and Root Spider can be a bane to aggressive decks. Those are our options to counter them:

Sinkhole can get rid of defensive sites
Lacuna Entity + Apply Alkahest can remove burrowed Root Spiders or submerged Coy Nixies
Atlantean Fate can silence defensive sites and additionally flood Root Spiders
Floodplain can act as protection against Root Spider, or outright destroy them if played adjacent by surprise
Mobility spells like Grapple Shot + Whirling Blades or Ruler of Thul allow to maneuver around/over those sites or minions
The deck features a high amount of 2 power minions that can enter Gnome Hollows
Don't be afraid to discard a card or sacrifice a small minion for the first Troll Bridge trigger in a turn. You can simply draw an extra site with your Avatar to discard it to Troll Bridge. Once a minion is on a Troll bridge, it can stay there and keep attacking enemy's life total.

Color requirements + coverage

This takes into account subtracting 1 of every threshold from our spells.

3 Earth spells (Autumn Unicorns) require 1 Earth threshold (+Gift of the Wolf from the collection) - 8 sites can provide earth thresholds + 4 Ponds can be used for the Unicorn
6 Air spells require 1 Air threshold - 11 sites can provide air threshold + 4 Ponds can be used for the Gyre Hippogriffs
19 Water spells require 1 Water threshold, 3 require 2 Water thresholds, 1 requires 3 Water thresholds - 15 sites can provide water threshold, 3 Floodplains can provide 2
It is most important to try and have 1 water threshold in your opening hand

King Arthur + Infiltrate/Mesmerism anti synergy

Be aware of the anti synergy between those cards. If you draw them in combination, simply keep one of them in your hand. Luckily, all of them are very threatening on their own and require your opponent to answer the thread (or lose). I think King Arthur is still very strong in this deck, especially with the high amount of cheated mana and a is good counter to enemy's steal effects.

Other Advantage of Elementalist

Allows you to play an increased amount of colorless Tempo/Utility sites: Sinkhole/Cities/Troubled Town
Allows you to play the best Tempo/Utility sites from 3 different colors: Boulevard of Bones, Towers, Ponds, Wuthering Heights, Garden of Eden
The aggressive nature of the deck allows you to utilize City of Glass/Souls/Traitors better than any other avatar: Your opponent shouldn't be in the position to attack your sites or deploy their minions there, but need them to defend.
Allows you to play a very versatile removal suite from different colors:
-> Pollimorph + Infiltrate as very good spot removals
-> Grapple Shot + Whirling Blades as mobility removal
-> Apply Alkahest as catch all and mass removal
-> Atlantean Fate as mass removal (and often wins games by itself)
-> Minion removal in the form of: Fey Changeling, Succubus, Lacuna Entity, Piper
-> Toolbox has a very versatile selection of removals/finishers/Combat tricks from 3 different colors.
-> Mesmerism as strongest minion removal in the game

Elementalist Hyper Aggro

@Sersch