I'm thinking of getting a Fallen helm and turning my Plate Mail into Volcanic. (The plate has a fire MAX UV on it. I would be almost invincible to fire.) I'm thinking about FSC here. If I wear Fallen, how badly does my damage to Trojans drop? I'm going to be using this and that. (Please don't question my planned weapon load out...I like both weapons.)
So, finally, my question is this: Should I be aiming for Fiend Med (at least) on the Triglav? Does the reduction hurt all the much?
There have been many projects to measure the effects of damage bonuses and penalties (for example Lancer Knightz). Depending on the exact wording, they estimate that each level of damage bonus increases your damage numbers by 4% to 7%. So you can expect your Fallen fiend-2 penalty to decrease your damage by about 10%.
Now, whether that 10% actually matters (by increasing the number of attacks per kill) is a different story. Try Zeddy's spreadsheet, to get an idea of the health of the monsters you'll be facing.