Elementalists have a slow early-game and progressively get better as you level, due to better damage and faster charging, moving speed, and moves. It might be frustrating to play them at first. They have high damage with their bursting spells and pretty good AoE + 1v1 damage depending on the build. There are issues with the lack of grabs, invincibility frames, squishiness, and the prevalence of interruptions (which cause you to stop charging spells and put the spells on cooldown).
With that in mind, here is some basic information:
T1s: Low cooldown, filler moves. Includes Florae Circle, Lantern Firebomb, Frosty's Head, and Pluto. The tiers correspond to prerequisites for higher tiers. 10 of these for T3s. You should max at least one of these.
T2s: Low-mid cooldown spells used defensively (and offensively in some cases), used to push enemies around. Often, you use these skills for SP dumping once you max out your chosen T1s and T3s. This includes Ice Wall, The Void, Fire Pillar, and Sun Burst.
T3s: Mid-High cooldown spells used for burst and is probably your highest source of damage. Whichever ones you choose should be maxed. This includes Arctic Burst, Lightning Strike, Halloween Buster, and Black Hole.
T4s: This includes your EX skills (Shining Ice Fence [60] and Flame Void Volcano [70]) as well as your awakening. Not many eles call these T4s, but I put them together because they trigger certain responses from anti-EX skill/awakening rooms. The EX skills do great damage and slows down enemies incredibly with Shining Ice Fence.
Buffs: Your chosen buffs should be Magic Festival, Elemental Burn, Disenchant, and maybe Ancient Memory if you have extra SP. All eles should max burn and DE, as they give Int that is constantly maintainable. I personally recommend max magic festival due to the cooldown reduction, faster charge time, faster casting speed, and general smoothness of play with it on (it feels terrible without it I believe).
Passives: Memorize allows you to charge for less time and cast faster. Mobile Cast allows you to move as you charge your spells. Both should be maxed. You should also max some of the masteries (the ones you choose, generally, but you might only have enough for say, two). You may max magical crit if you have the SP for it. You should not max the awakening passive (Elemental Focus) because you're disenchanting the focus marks to get additional INT, plus you can only put two focuses per enemy and they override each other.
Others: It is inadvisable to max ancient library because it has a high duration-cooldown ratio, and it doesn't give that much INT anyways. I also recommend against maxing shururu because we have a lower cap than witches and you need that precious SP to up your damage on your spells.
In short:
Must maxes:
Mobile Cast
Memorize
Elemental Burn
Disenchant (AFTER you awaken)
Whatever T3s you pick up (max black hole or else)
One or Two T1s (I recommend Florae Circle for 1v1 damage or Pluto for piercing through high mob density/ranged attacking)
Masteries for your T3s
Both Flame Void Volcano and Shining Ice Fence (EX skills)
1 Phase Shift
Suggestions:
Max Magic Festival
Max at least one T2 (I prefer Ice Wall, you can also go with Sun Burst or Void)
Extras:
10 Void for EX Void
1 Aerial Lantern Firebomb as a replacement for bombs (fire source)
1 Leap for outer and things like that
Potential Skeleton Build Recommendations:-------------------------------------------------------------------------------------------------------------------------
Two Element + Awakening:
http://arad.ofli.es/plan/elementalist#Ag...gbz60Fz7IK
Choose two elements and max their T3s and masteries, and max a T1. I suggest florae or pluto. Then max at least one T2 and then max Magical Critical Hit and other things or just another T2.
Full Build Example:
http://arad.ofli.es/plan/elementalist#Ag...QD0JUB0JYD
That would dump SP into magic critical hit. The Black Hole TP was also a dump, since I am still assessing the usefulness of newer TP passives.
Three Element without Awakening:----------------------------------------------------------------------------------------------------------------------------------
http://arad.ofli.es/plan/elementalist#Ag...IKw6wC0JUB
Add to a T2 or mastery.
Full Build Example:
http://arad.ofli.es/plan/elementalist#Ag...LQkgLQlQE=
My Builds (aka non-optimal):--------------------------------------------------------------------------------------------------------------------------------------------
Ice/Dark w/ Awakening:
http://arad.ofli.es/plan/elementalist#Ag...LQkgLQlAI=
Fire/Light/Dark w/o Awakening:
http://arad.ofli.es/plan/elementalist#Ag...PQkgLQlQE=
To be edited because I don't want to somehow not post this.
Any mistakes pointed out and opinions given are appreciated. Feel free to ask questions. I probably missed something because SP logged me out before saving my post...
Edit:
Tips/ Weird Stuff:
-Fully charge every single attack you are able without being interrupted while doing so (let go if you're about to get hit)
-Charge up your strong cubes(T3/T4) before you enter a room, and release it once you enter it. Then use T2s and T1s to clean up.
-Use T2s to move your opponents and do damage
-Ice Wall, Shining Ice Fence, and Astral Storm are the only elementalist skills that can't be charged.
-Use disenchant on enemies that have the focus mark on them. Aim to hit two enemies that have two focuses on them or four enemies with one focus on them, because the max number of buffs DE can disenchant is 4. This will give you 200 INT.
-Use Void in conjunction with Black Hole, and use disenchant whenever you black hole
-Ice Wall can block the explosion of room 2 in Goblin Kingdom
-You can leap over the bot explosions during his scripted attack (nenbots) in Rangelus's Guerillas in the boss room like every other class
- You and summoners are the ONLY subclasses that lack a grab and invincibility frames (I think Kasijas might invince frames during one of his attacks, not sure)