Heimskringla tells of how when king Halfdan the Black of Vestfold died his son Harald was still only a child of ten.
He split his army in two, one force led by his son Hake was to advance on land, while the other, led by Gandalv, was to cross the sea and attack the Vestfold army from the rear.
After the battle Guttorm turned his forces to face the army of king Gandalv that had landed in Vestfold, further south.
Guttorm forced Gandalv to retreat, and destroyed most of his army.
[1] - Snorre Sturlasson's Heimskringla translated by Samuel Laing (London, 1844).