Prestiging

It depends how much you need to prestige.
Sometimes there is an outlier at the bottom, with very low CP, with those above it in a tight CP bunch. In this case I will ignore the bottom one, and set my team up with the next few in mind - then kill the lowest fast and prestige against the rest.

Otherwise, if the CP are well bunched, then I set up a team that can take 3 or 4 in a row - hopefully not hitting a Blissey, and go max prestige as far as I can...then rinse and repeat.

It really depends on what is in the gym. If I have to I will just max prestige against the bottom mon...and then get out and go again - but this is my last resort.

It really comes down to the gym. Ideally, I would like to get through the entire gym, or as much as possible. I have 2 prestiging teams, one from 1100-1300 cp and the other from 1300-1900. I sort them a-z and and put "!T1" and "!T2" in front of the 2 teams to make it easier to select them.

My 1900 team can make it most of the way through most gyms. Sometimes I take it up to 2200 cp, mainly to use Machamp or a second Dragonite, but sometimes I can even use my heavy hitters. I prestiged a gym today where the lowest pokemon was nearly 2900. I just used my normal attacking team, all of which are under 2850, and rolled through the gym twice. That gave me the 8000 prestige I needed.

if the next is blissey or it is a pokemon which the CP difference between the bottom is just too high, i guess would be better to just prestige vs the first one

If I can get 1000 prestige with minimal potion use by repeatedly battling a single pokemon at the bottom, I'll do that. If there is a grouping at the bottom with similar CP and typing I'll fight that group.
I always shoot for getting 1000 point per battle... but it needs to be potion efficient. One way I preserve potions is by including "cannon fodder" pokemon in gym prestiging battles.

Depend on the gym prestige and avoid Blisseys, Snorlax won't be a big problem, though.

Assuming no Blissey or Snorlax, I try to use about two prestigers per Pokemon (so, aim to get through the bottom three).

100% depends on the gym make up and my prestige team. If I can get through 6 mons with my 6 mons and get close to 6k prestige then that is what I do.

If there are too many Blisseys I have no problem just battling the last mon for 1k or the last 2 for 2k if needed. Setting my line-up isn't that hard - I have my prestige team re-named for easy access.

Rename your prestigers with a 1 in the front and they will be at the top of the name list.