Minibosses honestly could use a bit of a boost in a general sense. Arcoris is fine, that one's difficulty rises properly over the course of the game, and Hunter is fine too (mostly), but the others tend to start out a bit easy, and stay that way.
They're not my design though, so I wont make any changes to them myself. That's all up to Pepisolo, he's the one that did the minibosses. He's busy with other aspects right now though that are more important/pressing.
Enemies like the Flak Launcher are indeed meant to be very hard though. They're categorized as "Fearsomes" in the game engine, and are designed to be the most dangerous things that aren't full bosses. Normal Demesne is probably the easiest to beat of that group; it's job is mostly just to complicate everything else, it's not very threatening once you've gotten rid of it's nearby allies. The others tend to just be designed to kill you.
The funny thing is, the Flak Launcher wasn't originally categorized that way. Originally, very early in development, I had misjudged it's difficulty (which is normal for me) and had it categorized as something much lower, a mid-level Wanderer (fairly common enemy type) if I recall correctly. I thought "this thing isn't too tough, this should do". Chris went "NOPE no no, nada, no sir" and upped it to Fearsome, which gave me an idea as to how far NOT to go, heh.