I'm sorry I have not had time to update this guide for all the changes to desynth. I can say with certainty though that raising skills from 200 to where you can reasonably desynth 255s is hellish. I've been trying HQ Tinker's Bacon with pentamelded 255s, which gives me about a 20% chance, and I've failed every single attempt. Rather disheartening!

PvP seems the best way, but it can only do so much when each match only gets you a single desynth, and you can still fail to desynth it. I don't think there's a better way though for LTW (i205 belts are your friend) or WVR. If PvP is possible for you, I'd recommend collecting a number of them, melding one materia to each, then downing a tinker's bacon for a near 100% chance. After a bit, you shouldn't need so many buffs (but you should still meld until you can hit 100% without it).

Regarding the experience chart I made being out of date, it's very out of date. They buffed exp from desynth massively since I wrote that.