I agree that as a stand alone spell 3d6 is not enough damage. 3d6+1/lvl seems more in line with a usefull spell. Even 2d6+1/lvl would be better. This way you are doing damage comperable to the rest of the party.
On the other hand, this is scaled to the fact that you will also have an animal companion that will be fighting for you, and you can now spontaneously cast any spell to make a summon natures ally, so you can have a small army fighting for you with a minimum of preperation. Then, hit them a few times with this spell as fire support for your troops.
This actually seems like a pretty viable strategy once you think about it. It doesn't scale in level, but the damage from your "pets" does scale with level, so this just helps you turn a wound into a kill as you see fit. The nice thing here is that your relatively long duration meens that you can use it over 2-3 battles in a standard dungeon setting. So the way you use this is cast it, cast a summon monster. Go into battle, when a foe is badly injured hit him with this for the kill, otherwise cast a different spell as you see fit. go on to the next battle. Rinse and repeat.
I actually think it is a good rewrite overall.