One damage is enough to one-shot most summons since most summons HAVE one HP. Furthermore, the fire mage "occasionally" gets 'Ignite' cards that deal 5 damage. And by occasionally, I mean he gets them whenever you place down high-HP summons.
And a "two turn insta kill" doesn't mean much in the scheme that it either guns down anything that gets close as well as having summons on top of that.