Strikes have a weekly rotating light-elemental burn, and two modifiers which rotate daily. Stasis singe is one of these daily modifiers, along with things like Grenadier and Heavyweight. Right now, the weekly singe is Solar and the two daily modifiers are Stasis singe and Iron. As per the pinnacle challenge text, 'Complete strikes using the same subclass as the currently active elemental burn', running Stasis when the Stasis singe daily modifier is active, will count towards the pinnacle.