Because this is a one-on-one battle, Gala should be well prepared in advance since Vahn and Noa will not be able to assist in the fight. Songi will usually attack Gala with a 4-hit physical combo that deals between 110-150 damage if all hits connect. He will randomly (and often consecutively) use his special attack Hyper Wave, which deals about 230 damage if Spirit hasn't been used and about 75 damage with Spirit.
Gala should have already absorbed Vera for healing purposes, as any healing items will want to be saved for the upcoming boss fight with Zeto. Ideally, Gala should be equipped with the Defender Chain accessory in order to increase the rate of blocking hits, as well as any defense raising accessories like the Azure Jewel. He should at least have 3 art blocks available by this point, so Thunder Punch should be used each turn (if it has not been learned from the Thunder Book I, have him use any 3-block Arts repeatedly). Spirit is not a necessity in this battle, as Songi has no predictable pattern of when he will use his Hyper Wave. But Gala should always use Vera to heal whenever his HP gets below 260.