Keep your distance and keep attacking? Normally a Dark/Dark brute is not someone to worry about in PvP, at least in my experience. Most likely it'll boil down to a test of endurance. You'll have it, and he'll eventually run out of it. On top of that I believe energy resist for DA is pitifully low, and that's the majority of your damage type.
Step 1: Try to plop an apocalypse chance for damage proc into neutrino bolt.
Step 2: Laugh maniacally.
It's definitely worth the cost. Force fields isn't a great secondary for a solo pvp defender, but radiation blast is one of the better ones, and you should be able to take out less experienced people on your own by leveraging it properly.
about uh... before I17 i had a subscription... well i do again now resubbed 2 days ago and around then i almost took down single handedly a dark/dark brute thing is he used insps im FF/Rad/dark with a decent build fully IOd dispersion bubble and this was before I17 :P any tips on how i can accomplish this again it was in sirens call :P