Yeah, I was planning on starting with Oshawatt and only using it. Since Timid, Modest, Calm, and Bold are all seen with either HP Ice or HP Flying spreads on Thundurus and Thundurus-T in VGC, I'm thinking I'll pick one of those Hidden Powers, find an IV seed for it, then find the number of Chatters necessary to hit each of those 4 natures (and the corresponding PIDs). That way, I can restart and RNG my ID/SID anytime for a quick and definite shiny with any of those natures without having to play through twice each time.
Hey. Thanks again for your advice on RNGing shiny roamers. If you don't mind me asking, when you did it yourself via the method you're suggesting, how long did it take you to play through and get to the Master Ball for Tornadus/Thundurus (or just to Tornadus/Thundurus if you traded a Master Ball over)? I imagine you were rushing through the game since you had to play through it twice.