There's no real established criteria for what a Mage is or isn't capable of in WoW, though; all the marquee Mage characters from Jaina to Khadgar to Rhonin have been able to use spells and abilities heretofore unseen as the plot requires - just by dint of having access to the right power source, prior knowledge, or experience(s). A lot of your examples are abstractions that are down to separation of gameplay and story, such as it taking time for Jaina to mass-teleport people away (a mechanic necessary for the RTS game and used again in the Hyjal raid in TBC). Jaina has also historically cast a fireball strong enough to knock a Scourge Necropolis out of the sky pre-WotLK (in the canon Wildstorm comics), Khadgar performed any number of insane feats (like leading the cast to teleport the entirety of Dalaran), combining all three classical Mage forms of energy into a single massive spell to destroy a metal dam and flood a Blackrock foundry, as well as the creation of an Artifact-tier ring for the PC using random relics from Draenor.
As for Medivh, he was pretty much constantly struggling against Sargeras' control since adolescence and almost always on the verge of exhaustion until he was more fully possessed by Sargeras before his death - and the spell he used on Arcanagos/Nightbane was made using a portion of his own very essence, the only to bypass the Arcane protection the blue dragon was using. That this might exhaust him is understandable given his already conflicted state, plus it turned Arcanagos into an undead wretch that persisted long after Medivh's death at the hands of Lothar and Khadgar.