It is not until Buffy starts to battle Glory (a hell god who has been banished to Earth) that we realise just how powerful Willow is becoming. This is first evidenced when Willow (with the help of Tara and some magic dust) casts a “Teleportation Spell” on Glory: to remove her from a fight that Buffy was losing. In doing so, Willow again encounters concern from Giles, who advises her that the spell was “dangerous for an adept at your level Willow”.
Further evidence of Willows growing power (and lack of emotional control) occurs after Glory destroys Tara’s mind. Willow seeks revenge, and after reading a book on “Darkest Magick“, goes knocking on Glory’s door. It is here that we first encounter the darker side to Willows character, as she uses black magic to cause Glory pain: Willow freezes Glory, then lashes out with powerful blue lightening. Willow also makes use of several other nasty spells in her assault against Glory: throwing of shards from a shattered mirror, throwing of a bag of knives and summoning of a serpent. Unfortunately, none of this has any lasting affect on Glory (as she’s a god), and Buffy has to intervene to rescue Willow (as Willow has tired herself out).