Fire Trap is probably one of the most power levelling skills in Path of Exile that still manages to be viable in the end game. If you like explosions and fire in your builds then you cannot go past the Fire Trap skill gem. Almost any class or build can use this skill to level with but in the end game it's suited best to Witches, Templars and Shadows specializing in traps.
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Basics of the Fire Trap Skill Gem
To kick things off Ill begin with a basic explanation of how the skill works. As it's name suggests Fire Trap is a trap skill, which is considered a special form of spell in Path of Exile. Essentially you throw a trap at the ground, which then casts it's secondary effects when an enemy enters it's trigger radius. For Fire Trap these secondary trigger effects are an initial fire damage explosion and then a fire based damage over time ground effect. The damage of both of these effects are not reliant on your weapon like attack skills are, instead they are tied to the gem's level, which can be seen in the table below. As such, items that increase socketed fire gem levels are one of the best ways of increasing Fire Trap damage.
How Fire Traps Work
It takes one second to throw a fire trap and when you do a 30 second timer begins after which the trap will expire and break. Each time you do this an 8 second timer begins to refresh that trap. You can throw down three traps immediately after which you must wait 8 seconds per trap for them to be available again. By default you can have a maximum of three traps of any kind set at one time, throwing an additional trap when you are at this limit disables the oldest one that is currently set. When an enemy enters the trap's radius the fire explosion and burning ground effect goes off.
The passive skill tree has a number of trap specific buffs that can affect these mechanics including improving trap laying speed, increasing the duration before a trap expires, increasing trap trigger radius and increasing the number traps you can have down at one time. The most beneficial of these to the fire trap are laying speed and trigger radius.
One thing to clarify here is that although Fire trap is considered a spell it is not affected by cast speed passives or the faster casting support gem, at least not in the way you might think. When you link faster casting to fire trap the tooltip DPS goes up, however this is very misleading. This does not mean you can lay the traps faster, only trap laying speed affects that. What cast speed actually affects is the time it takes for the trigger animation to play, which is miniscule by default. As such, don't be duped by the tooltip into thinking that your DPS is higher with faster casting on your fire traps, in practice the difference is non-existent.
The Inital Fire Explosion
So now, with the trap specific mechanics covered I'll cover the initial fire explosion and what you can do to augment it. The fire explosion is immediate damage and can be buffed through increased fire or global elemental damage nodes. It can also be buffed through passive or gear increases to spell damage. As it is also direct damage you can even use the support gems added cold damage or added lightning damage to increase the overall burst damage the explosion does. Reducing your enemies fire resistance through elemental equilibrium or through the flammability and elemental weakness curses can also help buff the damage of the initial fire explosion. The explosion also counts as an Area of Effect, so increased AoE damage and radius passives can be effective. The final thing to note is that this explosion also has a base 5% critical strike chance and can work with the chance to ignite from flammability and the support gem with the same name. Ignite can also stack with the burning ground effect so there is some potential to trying to ignite your enemies with the initial explosion. At level 20 the base explosion damage is 812 to 1218, so as with some modification it can deal a significant amount of burst damage.
The Burning Ground Secondary Effect
And now let's take a look at the much more complicated burning ground secondary effect. The first thing to know is that this effect does not actually hit the enemy, instead it is a area based degeneration. This means it cannot proc on hit effects, it cannot crit, it cannot have damage added to it and it cannot benefit from fire, elemental or spell damage. So what can you do with it then? The first thing is to buff it through increases to burning damage. Burning damage is available on the searing touch unique staff, as a the quality bonus on fire trap itself, through the increased burning damage support gem and through increased burning damage passive nodes. The next thing is to lower your enemies fire resistances in the same ways that you did for the fire explosion. And then the final thing you can do is to cast vulnerability on your enemies to increase the damage they take from damage over time effects. Another thing to note is that although Area of effect damage increases dont work on the burning ground DoT, increased area of effect can affect its size, which can help with keeping mobs in the flames. On a similar note you can increase the default 8 second duration of the burning ground through increased skill duration passives and through the increased duration support gem. This can also help with taking down enemies with larger health pools, assuming you can keep them standing in the flames.
What is the Best Way to Maximize the Effectiveness of the Fire Trap Skill?
The first thing to do is to decide whether you are going to try and focus on the initial burst damage, the burning ground or a combination of both. I personally think the best approach is a combination of both, though it can depend on if your build is trying to do anything else.
As such, I recommend straight off that you put your quality fire trap gem in a Searing Touch, preferably a 4 or 5-link one. Then you want to support it with a quality increased burning damage, which also adds chance to ignite. Then support it with increased area of effect to maximize the amount of mobs that you can hit with a single trap. Another good option is fire penetration, which will increase the damage of the initial explosion and any ignite effect created by it, though it wont increase the burning ground damage. And then you can add an increased duration support gem or instead you can actually socket in another fire trap gem so you don't have to wait so long for cooldowns while farming. Finally I recommend the pyromaniac and immolation clusters of passive nodes. In a more general sense you should stick with high elemental or fire damage and burning damage nodes. Avoid burn duration nodes as these wont affect the burning ground. Also avoid anything that mentions weapons in the description as these wont work with fire trap.