My two cents here. I utterly suck now, but I've been in gaming since the Atari 2600. Back then I had my full senses to play so I could complete games properly. I say this so I can explain my point. Now, I know that many concepts about gaming have changed since then, but back then, the concept of "playing the game to learn it" was pretty strong, and I think many players still do this to this day. They want to learn mechs by themselves without the input from a guide. Now here's where I believe the problem lies. Back then, most games were learned solo (most were single player games after all), so if you failed, it affected only you. Not such thing in multiplayer games, since failing means you take others with your failure. And this is where the "learn before entering" mentality comes from, I believe.
As for OP and the thread topic, I kind of understand OP's frustration. I mean, a mini boss one shotting people? wat? But my advise would be to dial the fury down a little. Maybe leave the game for a few hours and come back with a clearer mind. Despite what everyone around here can say, getting good at this game isn't the smoothest, easiest thing. This game may not be the most complex experience out there, but certain things may take some time to learn (and let's not get started with all the hoops needed to gear up). Even I with my time in the game still forget sometimes that SS, CC and interrupts exist, and only getting hit repeatedly (in game, mind you) serves as a reminder. I don't play with others anymore. I feel my time for that is over as I grew old and uncoordinated (I also get huge headaches if I do content that taxes my brain, so feh!). But I know mechanics exist and we have to learn them and go around them. Some may seem unfair, but after doing them a few times, I suppose they can be etched into memory.
Anyway, keep your head clear and don't let a game make you mad.