Hellfire is a mid to far range dragon who specializes in damage over time. I play him as an assassin or secondary offense staying at range delivering my damage over time fire breath to kill my foes. Hellfire isn't a tank but can take a few hits so he is a great choice for passive players.
His Q is a breath attack that does Damage Over Time or DOT.
His W is a long range skillshot that has AOE= Area Of Effect burst damage and DOT.

Since Hellfires are ranged, pick weapons that work with that play style such as Skyburst and Sparrowthorn. Hellfire also can quickly consume mana and is also not the most tanky of dragons, but he can take quite a few hits, stay at a distance and be conservative with your mana but don't be to passive. Also keep the damage over time on them, not your Fire breath, with the damage over time you don't have to spend to much mana, just poke them with your fire breath and let that burn for a while. Finally Fireball has an Area of Effect which can also light multiple enemies on fire, don't spam Fireball wait for an opportunity to hit at least 2 enemies enemies at once. Also in a 1v1 fight Hellfires will almost always be the victor do to their DOT damage and range, if you get in a 1v1 fight, play right and he will not be able to beat you and will have a tough time "trying" to run from you.

- remember guys, Hellfire is a great solo unit, one of the best maybe, but only if you use it properly, its great for soloing, and great for taking out structures, but if you solo its only good in 1v1 fights, i you see 2 or more dragons all commin at you, start running, try to find some kind of decoy to slow them down until you can get away or find help. Also note that the Hellfires Fireball ability has long range, and pairs good with Sparrowthorn, so if your still new to the Hellfire, trying playing from a distance as a sniper for added security.

Depending on whether you are an assassin type player or Secondary offensive depends on what weapon and runes to pick. As an assassin I take Sparrowthorn for its high damage and range, to take out turrets and snipe low health enemies, however as a Secondary Offense pick Skyburst for its zoning since it will keep enemies at bay and can hit multiple foes at once. Other nice weapon picks are Phoenix Familiar, Beggar's casting, Fracture Barrel, and Hammer of Sul. I have heard from Tex, that Bladebiter is a good choice due to it being capable of restoring health and mana on killing blows and that includes minions so there is also that weapon choice.

Assassin runes - should exaggerate damage and cooldown since you want to burst enemies and towers fast so you can leave before the enemy finds you.

Secondary Offense - Go with mostly damage but with slight durability and little to no mana sustain.
Example Lvl 1,2,3 Damage increase, Lvl 4,5 Max health, Damage reduction, Lvl 6 Mana regen or Mana steal.

Finally spells, Spells honestly are based on how you play but some recommendations are Heal, snare, elemental, reveal, blink, and iron hide. As I said these are all on playstyle but with Hellfire you might consider to take snare so you can easily land your ranged attacks and blink to escape or, if you have Skyburst to re position your Arrow Storm.