After trying this team out for 2 days, I found that heavy stall walls it to death.
I was fighting a team of Heal Bell Blissey, Cursed Body Taunt Jellicent, CM + Roar Latias, Spikes + WW Skarmory, CM Virizion, and Ice Fang Hippowdon
I, no matter what I tried, could NOT the get upper hand in that match.
I use Tyranitar, they'd send in Jellicent and stall till Cursed Body activated once they saw I didn't have Crunch and they eventually burned me with WoW.
I use Garchomp or Jirachi, they'd send in Skarmory and setup and WW me out.
I use Sigilyphm they'd send in Jellicent and Taunt me. With 2 CPs, Stored Power was doing peanuts for damage.
I use Rotom, they send in Latias and setup on me and Roar me out. Even if I send in Tyranitar, Latias would just Roar him out or setup on his weak Pursuit.
I use Gliscor, they'd use Hippowdon and Ice Fang me. And only Rotom can really switch into that Hippowdon because of it. T-tar & Jirachi fear EQ and Garchomp, Sigilyph, & Gliscor couldn't take more than 2 Ice Fangs. I tried to burn him with Psycho Shift, but he'd just switch to his Sigilyph counter; Jellicent.
Blissey only switched in once the whole game and that was just to take a turn to clear it's team's statuses.
It was just a revolving door of hard counters and I couldn't do anything about it. Maybe if Garchomp had Fire Blast, it wouldn't have been that bad, but he just doesn't have a move slot for it.
I think it would really be worth your time to look into a stall-breaker that could take Rotom's place. He feels like the weakest link of the team outside of battling Rain teams. And he's no real help against stall. He can Pain Split Blissey & Chansey, but they'll just heal it off and Toxic him.
Problem is, I can't think of anything that could be a good replacement for Rotom-W without taking away the major assets he gives to the team, especially since he's part of the core.
I'm sure you can think of something or just changing a Pokemon. I'll keeping testing new things for now.