Blissey as attacker
Answers
If you want to kill fast, Blissey is not a good attacker, it has a low att-stat.
If you want to kill potion efficient, it depends - if you have max-potions to spend, it is, if you have to heal with 20- or 50-CP-Potions, it is not.
If you want to deal as much damage as possible damage with one mon, Blissey is perfect for you. Its "damage delt until it faints" value is by far the biggest in game, making it a great attacker according to this metrics and a perfect defender too.
Time can really be an issue. Blisseys attack stat is half as high as usual attacker's, so it roughly takes double time to kill something. It will be impossible to kill another blissey in time, it will be hard to kill a snorlax in time, and for all others, you have to be patient.