ok, so I fought Alpha-RXIII during "Monster Tea Party".....

[Mister Torgue Flexington]

and he managed to drop these:

http://steamcommunity.com/sharedfiles/filedetails/?id=1315822362

NOTE: pay attention to the purple orb that appeared when it died

[Kobalobasileus]

Yeah... that's an Orb of Masamune. You can use them at Smith's Anvil to perform certain upgrades in the Way of the Wang DLC.

[SeriousCCIE]

yeah this is pretty common now to be honest. or at least not uncommon. you want them!

[Mister Torgue Flexington]

yeah, but the main reason why I took that screenshot, is to basically show that this dude does indeed drop Orbs, just not very often (like most early/mid-game Mini-Bosses). in fact, here's some other screenshots of other mini-bosses who occasionally drops them as well, when they normally don't:

http://steamcommunity.com/sharedfiles/filedetails/?id=1135177886

http://steamcommunity.com/sharedfiles/filedetails/?id=1119477852

http://steamcommunity.com/sharedfiles/filedetails/?id=1157988518

http://steamcommunity.com/sharedfiles/filedetails/?id=1100178971

http://steamcommunity.com/sharedfiles/filedetails/?id=1082614030

http://steamcommunity.com/sharedfiles/filedetails/?id=970453027

Now the only reason why I didn't include Toxitor from "Stop the Ooze" or Old Fart from "Flirty Fishing-Part 1", is that they tend to drop Orbs a little more frequently (but still nowhere near the drop rates from ones like Highest Priest or Lord-of-War)