I usually had the best luck going after Luca, then Alto, then Michel, stick with missile volley's, an dkeep moving. The trick to getting S/SS, is to take out all 3 before the Vajra show up (I think you have 2 minutes). If you don't, the upside is whoever's left will help you take on the Vajra, but you get a much smaller bonus. Also, once you do get to the Vajra, there's only one tough large Vajra to worry about, though there are a couple misleading weak ones.