PlatinumDude, as I said, we are talking about wild Pokemon here not competitive battling. Where would Raticate find a Flame Orb in the wild? Okay, I forgot about Hustle but anyway nothing comes without an expense and this one costs the accuracy. So it's not all good news. Raticate with Hustle has a chance but without it the damage dealt isn't much to kill her any time soon.
Also, Weavile doesn't have a chance especially now that I ran some calcs I can clearly see he can't hurt her much. First of all, Weavile can't use Brick Break as he can't learn it by any natural way(it's a TM).
Low Kick only 3HKO'es her.
Night Slash and Ice Punch 4HKO her.
Add Softboiled in and you get a long fight where Blissey is much more likely to win because of her higher stamina meaning that Weavile may even get tired and faint. Also, Blissey can damage him with Egg Bomb and Double Edge as he is quite frail. Last, let's not forget that Blissey can use Charm or Defense Curl which instantly make her much more durable.
I really think Blissey can beat Weavile.
By the way UV, Blissey learns Counter through breeding which is very likely in the wild. I mean, what's so weird about a Blissey breeding with a Breloom or something? To be honest, Blissey with Counter could take all of them easily because one Counter by Blissey is OHKO for anyone.
Also, considering that we are talking about Blissey then happiness doesn't have anything to do. It already evolved in Blissey meaning she had a lot of happiness. Now it's a Blissey and no matter how sad or happy it is, it won't change.
About the battles you brought up:
Zangoose vs Seviper - Zangoose wins. It has Immunity so Seviper can't really do anything to it while Zangoose dices Seviper with its claws. I mean, snakes only have their poison and since Zangoose doesn't bother then it can win. It's like mongoose vs cobra in real life. Mongoose wins.
Fearow vs Pidgeot - Fearow for me. It's much more powerful and its beak is huge and sharp.
Aerodactyl vs Bastiodon - Aerodactyl really can't do anything to Bastiodon as he is so durable. Bastiodon on the other hand, although offensively weak, can damage Aerodactyl with Metal Burst. I just can't see how Aerodactyl could hurt Bastiodon - Aerodactyl can't use Earthquake as it is a TM and those are wild battles.
Sharpedo vs Wailord - Uhum. We are talking about a small shark versus a ginormous whale. Wailord wins. I mean, Sharpedo can't kill him not only due to Wailord's high HP but due to size difference alone. Although Sharpedo is supposedly able to kill Wailord after 4 Crunches, I can't see how this would be done. Anyway, going with base stats Sharpedo wins. Going with logic, Wailord wins.
Gyarados vs Milotic - Milotic may win. It is quite bulky and has Recover. Also, Gyarados can't learn any moves capable of killing her naturally because they are all either weak or resisted. Milotic couldn't hurt him much either but at the very end of a very long battle, Milotic would win because she can recover damage while Gyarados can't.
Gengar wins.