I wouldn't use heavier gear all the time. Rather put it on for specific fights because you can become immune to melee with some of them. That's not worrying at all about how much damage you will take. Yes - Balor will hit you for 0 on a 3 Con psicher
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I see no problem in using Vanishing Powder Grenade. You can't use them all the time, plus they aren't reliable as I've observed them not putting a character into stealth. For the Magnar fight in specific I think you can use your stealth to remove the few guard before the corridor and then just rig it with bear traps and gas 'nades. Then your NO will one shot him.
You can do something similar for the Goliathus beetles - make a field of toxic gas and watch them go down.
No EMP3 for Nevil
? With a decent Infused Hopper Leather and Tabbis you can run to gates and catch individual groups before they even come in the main room. You don't have Grenadier so you probably won't be able to save him, but you won't have great problems, either. Them Nagas you one shot are a good source of SS.
I generally avoid the Marsh, tbh. I don't much like the area and I don't consider there being something that worthwhile to go through all the fodder. Abandoned Waterways is much better for the loot.