Porygon2 to kill Blissey
In order to beat Blissey before time runs out one needs to put out damage as fast as possible. The key to this seems to be a Quick move with high EPS loading a Charge move with high total output. Porygon2 has Hidden Power, with in 1.5 seconds produces 15 energy and 10 damage. It charges the bar in only 10.5 seconds, providing 70 damage during this. Follow with Hyper Beam, one bar, 3.8 seconds, 150 damage. In total 220 damage in 14.3 seconds, for an overall DPS of over 15.
With STAB, which both Ancient Power and Hyper Beam have, this goes up to over 19. Porygon2 also has a decent base stat of ATK 198.
Other viable Blissey killers by this principle: Piloswine with Powder Snow and Avalanche or, maybe the best, Gengar with Hex and Focus Blast or Sludge Bomb.
Answers
I was lucky enough to evolve a perfect-IV Machamp with C/CC. Powered it up to ~2550, attacked a 2900 Blissey, didn't dodge ZH's and missed 2 dodges on DG's (which are spammed really quickly when Blissey is taking a lot of DPS), and died horribly. Next try I was careful, dodged everything, and killed it with about 40 seconds left, and maybe 20% of my health left. Hope this gives you an idea of the best-case scenario against the pink menace!
I gave the following answer to a similar question today:
To avoid timeout, high DPS is required.
The following deal the most damage, so you should use them even against Blissey with Zen Headbutt + Dazzling Gleam:
Machamp: Counter + Dynamic Punch
Dragonite: Dragon Breath + all charge moves
The following Attackers have high DPS. Among those is a preference depending on Blisseys moveset. Most of the highest DPS attackers happen to be Fire or Psychic types (and in one case Rock/Dark), which have a slight type adventage:
Against Blissey with Dazzling Gleam:
Flareon: Fire Spin + Overheat
Charizard: Air Slash + Overheat
Against Blissey with Zen Headbutt and/or Psychic:
Alakazam: Psycho Cut or Confusion + Futuresight
Espeon: Confusion or ZH + Futuresight
Exeggutor: Extrasensory + Solar Beam
Against Blissey with Pound or Zen Headbutt + Hyper Beam or Psychic:
Tyranitar: Bite + Crunch or Stone Edge
Of course I only mentioned the preferred movesets, but the following moves are also serviceable:
Machamp: Counter + Close Combat
Dragonite: Steel Wing or Dragon Tail + Outrage
Edit: Heracross: Counter + Close Combat
is as good an attacker as Machamp. I did not add him, because he might be regional, or has this already been cleared up?
Heracross with Counter/Close Combat is my favourite Blissey killer by now, although I need more of it because he always get killed (but an Exeggutor with ZH/SB does the rest of the job in time). Or a decent IV Heracross to power up. Mine has 2200CP and was able to defeat a 2100 Blissey alone, but above it, I use Exeggutor as the 2nd mon.
EDIT: I live in Brazil, so Heracrosses everywhere. Already got 50+ candy