Pyros were always among the nastiest, most ruthless survivors of Cryos's extreme wastelands. Evolved to possess minor flame-making capacities, they "blossomed" under Darkspore mutation into full-blown pyrokentic monstrosities. They unleash cones of fiery death against any foe or group of foes, although such attacks have limited range.
Pyros are no threat at long-range, but everything changes when they get close enough to unleash their fiery breath of death. Beware the pyros, keep them at bay with attacks such as Sage's Strangling Briars, then continue to fire upon them from a distance.
Pyros are very dangerous when they are found in groups, so be wary when approaching them. Attacks like Vex's Time Bubble also works in a similar function to Sage's strategy, but Tempests are probably the best choice when fighting pyros.