1) Yes, Corsairs are highly sought after right now for more current high number end-game content such as Legion and Provenance. They are often used in conjunction with Bards to significantly boost melees, and even without Bards, they are still highly sought after for the sake of party buffs.
2) The weapon you're referring to, the Empyrean gun Armageddon, is arguably in the line of easiest Empyrean weapons to make. To get the level 85 version, which is the minimal level you need it in order to obtain the Wildfire weaponskill, it involves a certain number of Notorious Monster kills all across Vana'diel, but as far as the actual Abyssea NMs you need to farm, Carabosse and Cirein-croin are among the easiest to farm. They only require two Key Items a piece (which can be farmed from gold pyxides alongside actually farming the NMs) and could easily be hammered out in a short frame of time by a melee and a WHM. Over Thanksgiving, myself and about four other linkshell members finished up a number of Carabosse for two Armageddons being made, and then we farmed about 80 + Cirein-croin Lanterns in the span of maybe 6 or so hours. I'll say that if you have the time and the drive, you could easily get 50 Carabosse Gems and 50 Cirein-croin Lanterns in a day.
As for its use, it is basically the staple weapon for Corsairs nowadays. The +AGI on the weapon is a modifier for Quick Draw, and Corsairs have a combo that significantly boosts Wildfire damage. The Navarch's Bottes +2 (Corsair's Empyrean +2 feet) enhance the effect of Quick Draw by causing the target to take 20% more damage from attacks of the last Quick Draw element for a frame of 15 seconds. For example, using Fire Shot will cause the monster to take 20% more damage to Fire based attacks for 15 seconds. So using Fire Shot with the Empyrean +2 feet will result in Wildfire doing 20% more damage than it would have before.
3) It depends on the situation, but /Whm, /Rdm, and /Sam are pretty much the most common used Subjobs for the content I mentioned in 1). That being said, it is good to have flexibility in what you can sub, so having /War, /Nin, etc available is something that's not going to hurt you at all. Nowadays, flexibility is important to have, and if you can't be flexible in your job, then at least be flexible in your subjob options.
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