Best Attacker against Blissey
To pick your attacker, I have preared the chart below.
If you want to damage the Blissey as fast as possible (and it does not matter how long your attacking pokemon will survive), you want an attacker which is high on the left side of the chart:
Machamp, Heracross, Ursaring, Dragonite, Gengar, Flareon, Charizard.
If you want an attacker which makes a lot of damage during its whole life time (and it does not matter how long it takes to take down the Blissey), you want a attacking pokemon which is high on the right side of the chart:
Blissey, Donphan, Snorlax, Tyranitar, Lapras, Dragonite, Vaporeon.
You probably want an attacker which will be a good mix between damaging the Blissey quickly while not fainting too fast. You may then look at the middle of the chart:
Machamp, Heracross, Dragonite, Tyranitar, Ursaring, Charizard, Flareon.
Answers
It is a little complex. Hence the explanation rather than the axis definition.
X axis: how much is tankiness (DefHP) taken in account in evaluating the attacker.
0: only DPS is taken in account
50 : DPSTankiness^0.5
100: DPS*Tankiness
Y axis: What is the value (in %) of the attacker compared to the best possible pokemon for a given X value.
I normally use Rhydon with RS+EQ/SE or Vapeoreon with HP but they need to be almost on CP par to achieve this.
Enter my lvl 33 Executor with CF+SB! - Confusion does the highest quick damage and Solar Beam the most charge damage.
I have brought down 2900ish CP Blissey (dodging charges) with my 2600ish Exec with CF+SB -
Exec is my go to mon to solo high CP Blissey without fainting!
Smashing Blissey one-on-one depends upon the moveset it possesses. Not even big bulky mon's like to eat a Blissey Thunder Wave or Ice Beam. For example, The latter can freeze Rhydon to death and Psychic fries Gengar.
In most cases high IV/CP mons with Focus Punch can destroy Blissey. Likewise in most cases Explosion can nuke Blissey, but not Electrode's Explosion.
Fighting and Ground types do okay but in my book, until Zapdos comes into PoGo, Blissey's current number one counter is a max IV/CP Celebi, especially if you employ dodge.
