If range is line-of-site, your infantry men could be parked at high elevation to taser the enemy from far beyond archer range. Defensively, they could be stationed on high walls and towers
Catapults could lob jars of oil, that these units could light after delivery from any range.
As infantry, these people could break shield walls (because they can taser the person holding the shield) and break lines of pike men (because their power reaches past the spear). In these formations, it's vital that every soldier maintain their place (which they can't do if zapped to the ground and writhing). This would make them an alternative to cavalry (whose job is the same thing, but they expose themselves to personal injury to do it). These could also be used to open a hole in a pike wall to allow cavalry through to rush archers or demolish dismounted infantry.
This kind of unit could also be useful as anti-armor. As Game of Thrones well showed, a knight in full metal armor is nearly untouchable by anything except a set pike, lance or javelin. Your pyromancer could just zap the armored unit and he is down, where weaker infantry could pull off the armor or drown him/her in muck.
This unit could also be excellent anti-cavalry, either zapping the mount or zapping the knight off his horse. Since cavalry move quickly, they are very challenging for anyone else to engage, but your pyromancers could dismount them accurately with a glance.
You could also use your pyromancers as anti-seige. They could zap enemy soldiers on ladders, keeping them stuck and blocking their comrades from ascending. They could zap whatever is pulling towers (provided it is in line of sight), zap catapult/trebuchet crews, light enemy oil while it is still in the trebuchet (causing explosions in the enemy ranks).