Fire Elementals...

[Gratuitous Lurking]

I think the biggest issue I'm having right now is due to fire elementals, and I'm honestly not sure what I can do to counter at this rate. My defense lines seem to be getting torn apart every time they mass, and even with a lucky new moon worth of time to prepare, ready ballistaes and a bit of a choke point defense, it seems like this flame-throwing gits just break their way into two sets of stone gates, and when that happens, I'm basically boned to watch my inner defenses struggle and hopefully survive for the night.
Is there something I'm missing here? Upgrading to gain better building options, or just not massing the right sort of things? I'm almost literally spamming ballistaes behind curtain walls combo'd with magical-based turrets at the end of a corridor, but it doesn't seem to stem the tide of damage, not to mention the constant burnination of my peasents. At this rate, I'm close to yet another restart -3-

[DragonAtma]

Grab them, then drop them in the middle of the map's biggest lake/ocean.

[...👑 JOST AMMAN 👑...]

Fire elementals are the most dangerous monsters in the game, so it's normal to have trouble with them: you need to use the Banish spell at the right time to throw them out before they destroy your towers...

[Lemures]

You should be encouraging them to travel one way and not letting any peasants tackle them at all. Stone ice golems and crystal ice golems along with ice ballistas. Although they are ranged a lot of defences do still outrange them so if your towers are not beside the walls you can use regular stone ones and work them down.

Also set up a cullis gate/lightning rod system near the frontline but away from villager traffic so you can grab a bunch of the gits and drop them into the pot for more essence flow. Allowing you to use more frost auras and magic before they reach your outer defences. You shouldn't need any gates, it just aggrivates mobs and slows down your own people/golems.

[DragonAtma]

I do use gates, but merely as maze shortcuts (monsters can go around them, but it takes longer).

It's similar to the shortcuts in IKEA.

[BrendyBear]

The fire elementals have a range of 8 (unless its been changed, although this is my experience in the game lately) and have a weakness to ice. Putting ice based towers, like ice arrow or ice elemental towers, 9 squares from the wall should destroy them pretty easily without being damaged at all. Ensure your maze is long enough that the towers can kill all the enemies before they even get close to the end. The monsters should never be able to attack your villagers, except during blood moons (lightning golemns work well for that).
The banish spell is a bit of a double edged sword though, depending on your layout. It is extremely effective at stopping the monsters, getting rid of large groups and preventing any more from moving through that area for a period of time, but it will drop the enemies around the map randomly, which can cause them to come from unexpected directions. I tend to only use it as a last resort, but it is effective at preventing the fire elementals (and anything else) from destroying things if you have a breach.
Cullis gate + lightning rods is brutally effective, and gives you loads of essence, but late game the numbers of enemies are so high that you'll only be able to kill a small number of them, even if you target the fire elementals. Its a great way of getting more essence, and will be sufficent to kill waves by itself for a while. The Ancient Cullis gates on maps are perfect for clearing the early days.

[Daynen Drakeson]

If you have enough influence to banish something, you have enough influence to pick it up and drop it in a cullis gate. The banish spell is probably the worst one in the game and WILL get you killed sooner or later. As long as you have two or three gates and a couple lightning rods, you can drop in every elemental that bothers you...and then mend your rods and gates before they pop.

Dropping them in water also works (never tried it, but makes perfect sense--DAMN this game is so wonderuflly nuanced) but I'd just as soon drop them in a gate to get the essence faster.

[Tiberiumkyle]

Banish is good for emergencies, can buy you a bit of time while you get something longer term sorted, hopefully.