There are Dengar teams soloing the raid, but no Dengar doesn't mean you won't get resisted. Tenacity down negates all tenacity, but not the base 15% resistance.
I believe somewhere I read that Rancor had 100% tenacity, of course this could be wrong... But if you, and you have 85% or higher potency, then tenacity down isn't doing anything for you.
What Dengar does allow you to do is concentrate on more speed than tenacity for other toons, so they get more chances at tmr. He calls assists for more tmr. He also has the only slow that can last through the door phase.
He's a great utility toon and works well in many encounters. I've used him in raid, GW, and arena to great effect. Combined with Phasma VM he is excellent in group encounters guaranteeing his Aoe does Ten down, DoT, and 50% tmr. With Phasma lead I've had QGJ call Rey to assist, Phasma lead call Dengar, and then Dengar call Rey a 2nd time for 60k damage total on one round.
He has put me from doing decently soloing p1, to not even having to think about p1, and if Teebo's damn stealth bug didn't rear it's head I'd be soloing p2/3. So far I've gotten 3.2 mill with Teebo, qgj, Dengar, GS, and switching between ee/phasma as my top runs when my best before Dengar was 2.4.
