I spend a lot of time trying to figure out who would win in a fair fight between the assembled Volturi Guard, and the Denali-Cullen-Werewolf alliance. Edward, Alice, Jasper, and Emmett are four of the best fighters in the world. I'd say the four of them could incapacitate Felix and Dimitri with ease. The twins would be harder though. Psychic powers and raw strength doesn't mean much when Jane can knock you out from across the room, or if Alec can blanket the battlefield in a toxic fog. Of course, Bella would be a match for Alec and Jane, but Felix and Dimitri would have no trouble killing Bella. And that's just looking at 5 Cullens vs 4 Guard members-- Kate's not a bad fighter herself, and the Werewolves are lethal. Even if the Volturi outnumber the Cullens, it would be a very close fight. It all depends on if Dimitri can get to Bella before Bella gets to Alec, I'd say (Alice and Edward could probably kill Jane if they mind melded, since Jane can only torture one person at a time)