My favourite pyromancies are Great (Chaos) Fireball, Great Combustion, Power Within, and Black Flame. Fire Tempest is also really good against large enemies and bosses. The first step for pyromancies is Laurentius of the Great Swamp, who can be found down in The Depths. He gives you a pyromancy flame (unless you started as a pyromancer) and sells the lowest level pyromancies, as well as upgrading your pyromancy flame. Once you get your flame to +10, Quelana of Izalith will spawn in the Blighttown Swamp, provided you haven't killed the Bed of Chaos yet (highly unlikely you will have done seeing as it's a late game boss). She'll appear between the tunnel to Quelaag and the bonfire in the swamp, on a patch in the middle of the swamp. She sells the higher level pyromancies, like Great Combustion and Great Fireball. Power Within spawns in upper Blighttown, but is very situational. Black Flame can be found in the DLC if you have it. The invader you fought could have been using a modded character, unfortunately they're quite common. The PvP range is strange, it depends on the method of invasion. Typically, red invaders can invade anyone a higher level than them, and 10% of their level below them (e.g. a level 50 will be able to invade anyone level 45 or above). Blue invaders use a different formula, but can only invade guilty players. They can invade anyone 10 levels + 10% of their level above them, and anyone 50 levels + 20% of their level below them (e.g. a level 100 can invade guilty players between levels 30 - 120). If your invader was blue, they could have just been a much higher level.