I know this has been done but I'll do it again because people kept bringing this up in the viability rankings even though it's completely outclassed:
Don't Use This!
Sure, Typhlosion may have gotten Flash Fire, but that still doesn't save it from being outclassed by other Fire-types in RU. Flash Fire is just as situational as Blaze is, and while it has Eruption, it must be at full health to take advantage of it, and its SR weakness is not helping it. In addition, its only average speed and only average bulk leave it vulnerable to being revenge killed. It can even be killed by the mon it's trying to take out because of Eruption's failure to OHKO something and the fact that Eruption gets weaker as Typhlosion takes damage. Its mono-fire typing and rather meh movepool limits its versatility and limits it to choiced sets. The only coverage other than HP that it gets is Focus Blast, and unless it hits something for SE it isn't doing much, especially if running Scarf.
Use One of These Instead:
Simply put, if you want a special Fire-type, then these three are the way to go. The main thing that outclasses Typhlosion is Houndoom, but all of them have something that makes them worthwhile using. Delphox has Psychic STAB, Calm Mind, Switcheroo (great for Choiced sets), and Grass Knot, allowing it to hit stuff like Qwilfish, Rhyperior and Slowking that can take an Eruption. Meanwhile, if you'd rather have more bulk and are willing to give up your speed, or just want to use Trick Room, then Camerupt is a good option as it has Sheer Force to boost its power, Earth Power to hit any Houndoom, Camerupt, Tyrantrum, Qwilfish or Rhyperior thinking they can switch in, as well as Stealth Rock to support the team. The real kicker, however, is Houndoom, which also has Flash FIre. It is capable of running a Nasty Plot set to sweep late game, Dark STAB, priority in Sucker Punch, the ability to Pursuit trap, and it can even afford to run Destiny Bond to act as an emergency stop to sweepers or take down one last mon once its job is done, or stuff like Will-O-Wisp or Toxic. All three have consistent STAB, and higher Special Attack, and have their own ways of negating priority: Delphox resists Mach Punch, Camerupt is so slow and bulky that priority doesn't really do much to it (aside from Aqua Jet by why use that on Camerupt), and Houndoom is so powerful that Hitmonlee, the best Mach Punch user in RU, can't switch into Houndoom. Even mixed Emboar is also useful if you don't mind having a physical attack in your moveslot (what can switch into it, anyways?).