Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - Enhanced Edition Secondary Skills for Bairdottir?

I've decided to run a party of:

Knight (Man at Arms)
Inquisitor (Close-range spells from several magic schools)
Wolgraff (Scoundrel/Aero)

And Bairdottir as a Ranger.

..But should I give her points into a magic school as well for utility? What would be a good one? Hydro for healing and rain? Pyro for Burn My Eyes and Wildfire (Haste)?

Or just leave her as a single-classed Ranger?

Hydro and Pyro are valid options for the reasons you provided.

Alternatively, as the Ranger is Dex based you also could add one or two Scoundrel levels. There are many Novice Scoundrel skills a Ranger can profit from, e.g. Fast Track to haste her without Wildfire's heat effect, Winged Feet for positioning over/on harmful surfaces, Hide in Shadows for protection from advancing enemies, to name a few.