Using Mystic Forge for Incenerator?
Well, what I did when I was going for my legendary was to just throw in whatever rares I found. I wound up with A precursor at the end of the day (in this case Zap), and was able to effectively trade it for the one I wanted through the Trading Post.
personally I have been saving money, I have spent over 300 gold in rare swords, and about problably the same amount of gold/time in self crafted ones. At some point I realized that instead of getting pearl exotics, I could have used all that gold to buy the weapon straight up from the TP.
I know of cases of people who have spent much more than I have. I also know cases of people who get precursors quite often. RNG will be RNG. Safest bet, and out of experience; farm the gold, buy off TP.
Legendary SoloQ
Throw exotics, way better than rares.