I have a dark*3 sent and its fun as hell, but you seriously need to engage brain to play it. I decided to build it like a buzzsaw and ignore survival beyond basic slotting of the resist shields and getting basic defense from the powers already there. I ended up with 20-22% def to all and that is it.
what my build does have though is plenty of PBAoEs and ST attacks with procs out of the wazoo. I guess its kill or be killed, which isn't an intelligent way to play sents, but is is a way to play sents.
You realllly have to lean on the Nuke -ToHit debuff and aim to kill everything that is a serious threat within that debuff window. Nuke+PBAoE from Dark (Engulfing darkness?) and Dark Oblit is enough for the little baddies and can mostly sustain through the bosses.
The only time i really felt on the edge was a BP mission in one of hte lvl 50 arcs in PI. Albeit was running it at +2x8 i think, so probably a bit ambitious anyway.