Just beat it with my monk, and the language isn't clear — is investing in the guardian pet stat guardian's range going to enhance the range at which the little pet reaches out to boost towers, or will it enhance how much range boost the towers he can reach will receive?
It will increase the range at which the pet reaches out to boost towers.
The other Guardians also have a guardian range stat although their effects are to increase stats other than tower range (damage, attack speed, and resistance).
There should be a second stat you can invest points into that increases the effect that the guardian has on a tower if it is within range. For the Apprentice Guardian, that stat was right beside the guardian range stat.