I just got it Masterworked by using the Castellum area of the Leviathan raid. Took me something like 45 minutes. The very best/quickest way is to just get to the Castellum, trigger the enemies, and then get on top of that huge platform that has those huge golden statues on it (you have to jump onto those torch towers with the blue flames to get up there). Then wait for the enemies holding shields to get into position. Move yourself so you have an angle on two or three of these shield carriers. You can proc the Perfect 5th shot simply by two-tapping two of the shields, over-and-over. If you mess up and do damage to an enemy without killing it with the Perfect 5th bullet just use that enemy to get a couple critical hits. The whole trick of this method is that at 385 one headshot on an enemy with the Perfect 5th bullet will take down the health of these Cabal to like 90% and then the BURN EFFECT that lingers will finish the enemy off--AND THAT'S THE KEY, HERE! This is by far the fastest method. You proc your Perfect 5th bullet off of the shields, then put that round into the head of an enemy. They will burn to death and that will count as one kill for the Masterwork. Just every so often stop this to clean up the half-wounded enemies that will start to collect from splash damage. Fresh troops will come out to replace them so that you can move your crosshair quickly from the shields to a 100% health enemy. Rinse and repeat. Once you get in the groove you'll have your 50/50 Perfect 5th kills in about 20 minutes, at most. And then you just have to get something like 500 kills to satisfy the first requirement. Once you just go ham you'll kill 500 surprisingly quickly... maybe 20 minutes more. Tip: just always try to get crit shots off of enemies and off of shields because each crit puts a couple bullets back into the mag. I only had to jump off of the big platform and collect ammo once during the whole process. Try not to reload, either. It will take soooooooo long to run out of ammo if you get most of your crit shots. Do not go anywhere else to do this. Trust me. Set yourself up like above. It's crazy how fast and easy this goes. The shields work so good! EDIT: Masterwork adds DRAGONFLY to the weapon which is then on top of the explosive 5th shot after getting four critical hits. The fact that you get so much ammo back from critical hits makes this Exotic quite amazing. If you already have exceptional Legendary kinetic and power weapons in your arsenal this Energy weapon truly carries its weight--too bad you couldn't also switch between elements but that's asking a lot.) Cheers.

Mess Sean put out a good youtube video on it, fire up the raid, go in solo and follow his instructions

Only kills from the explosion's burn counts, not the explosion itself.