What happens with your PC when you try to do dungeons? Does it crash the game completely and kick you out before you can complete a run? Or does it balk at updating the screen so you can't see what's happening around you? Fail to respond to controls?

The first time I ran Sastasha, I hadn't figured out how adjust my settings well, and I could barely see what was happening due to all the flashing flaming graphic effects obscuring the screen. I couldn't see what was targeted, so could only rarely get in range to perform an actual attack. I wound up just getting somewhere in the middle of the mess and spamming Flash over and over. Not the ideal way of tanking a dungeon, but for a simple one like that, it actually worked out ok (though I needed to pause a few moments between mob groups to let my mp levels recover).

Have you asked your FC for help? There's only three fairly easy dungeons (levels 15, 16, & 17) before you gain access to retainers, and one primal fight (level 20) after that before you gain access to a chocobo mount. If your PC isn't kicking you out of the game completely, a helpful group who knows what you're dealing with should be able to help you get through that much, even if you are stuck spending more effort on fighting with your computer than fighting with the mobs.

(Once you get either the PS4 version or a better PC, you'll want to go back and learn how to tank properly with a working system. You'll find the game a whole lot more fun once once you can focus on the in-game challenges rather than interface ones.)