IMO , the first unit you will awaken is either a healer , archer or tank since they are more general use then a duelist. Personally , i'd say to keep your spirits untill awakening comes around and you decide then what you will need to awaken first. Especially since those spirits are pretty rare and even if you had an infinite amount of them , the resources to awaken 1 unit fully (both the unit , and after the skill) will require you to farm for countless weeks.