This thing really doesn't get enough credit. With the fire rate mods stacked and enough crit chance to near-guarantee a crit every time, it's a monster. Obviously it does not excel at crowd control because even if you forego damage for more fire rate, it's still not going to fire very quickly, but it more than makes up for this in how few shots it takes to kill beefy enemies, and with a decent fire rate, it may not be anything close to the fastest way of dealing with crowds, but it will still take down more enemies than you probably expect it to.

I originally had [Speed Trigger] instead of [Vital Sense] here and said it was because the stats showed [Speed Trigger] doing a little more damage. This is because I can't read. Vital Sense in fact results in more sustained damage as well as single-shot damage. [Speed Trigger] is still a valid choice as it will bring the fire rate up to nearly a shot every two seconds, but Vital Sense will keep Diriga doing one-hit kills across the map at higher levels, which I prefer. Rate of fire is still faster than once every three seconds, too, so it's not a huge difference.

I pulled a Riven that adds Damage and crit damage and actually replaced [Serration] with it since it's damage stat was higher, to say nothing of how ludicrous it is to get crit damage above 7x. The real prize would be a riven that increases fire rate, but figuring out what mod to replace is a bit tricky on this one. If the Riven also has +crit damage, it may or not be higher than [Vital Sense] itself, although as long as it's relatively close, Vital Sense may be the best one to replace. Alternately, if you think of a +fire rate Riven as an easier way to match the non-Riven build's fire rate instead of adding more onto it, you can replace [Shred] or [Vile Acceleration] depending on what the other bonus is. If the the other bonus is +damage, +crit or +crit damage (or, RNGesus willing, any two of them,) replacing either of them with it will be a serious buff even though you won't have the visual satisfaction of watching the balloon fire more often.

I'm assuming Galvanized mods work on sentinel weapons, in which case [Galvanized Diffusion] should be fine since [Diriga] will probably take less than 20 seconds to get its next kill unless you're at like, super max level steel path.

Alternate Build
There's an alternate way of building this gun to get even faster fire-rate without sacrificing damage, but it requires a Riven with a specific stat: drop all three fire-rate mods, and replace two of them with Depleted Rounds and a Riven with -Magazine Size. You need the Riven because Depleted Rounds will bring the magazine down to 6, but the goal is to bring it down to 1, at which point the fire rate will become the reload speed, which is two seconds. Obviously, [Primed Chamber] is a good choice for the newly free mod slot.

EDIT: I've heard varying things about whether or not this actually works or if reload is a non-stat on this weapon/sentinel weapons in general, both in-game and through forum digging. If you've got a decent riven with fire rate and other goodies like crit-chance, I'd stick with the fire rate build.