There is no real set way to take the exams , however as you know you cant go solo until you've passed air law . The only other aspect that has an exam consideration is Nav as you cant fo your Qual X Country without the Nav exam . So Nav would be the better choice , this is also as now you have been solo you will starting that phase. All the other exams must be taken before your skills test so you can do those anytime . I would get Air Law and Nav out of the way first