When training a gym, do you

It depends on how many defenders there are and what the cp range is. If there's only 2 or 3 and I can realistically beat them all with half cp Pokes I might carefully select, but if there's a big gap between the lowest defender and 2nd defender I'll probably just work on the first one and pad the other attacker slots with random low cp Pokes.

Number 3 most of the times, gyms around me are normally level 1-6, using this way I can normally beat all the mons in the gym and not only prestige it to myself but leaving it with an available spot or almost with the prestige to get a spot.

My choice: always attack the first defender with attackers at max half CP of that defender then run away. That's the quickiest way to earn 1000 prestige points.
1) If the first defender's CP is ~1000 and below, I 'll choose any pokemons in my box: rattatas, pidgeys, poliwags... If they faint, I 'll transfer them, cost me no revives or potions
2) If the first defender's CP is above 1000, I 'll choose the 1st and 2nd attackers with type advantages, rests are random: pidgeys, rattatas... So I build an attacking team with wide range of types, A grade for attacking movesets and CP from 800 to 1500 for these uses: Starmies, Pinsirs, Scythers, Jynxes, Electabuzzs, Tangelas, Seadras... If they faint, I 'll use revives and potions to use them again.

Kind of 2). I sort by CP and scroll down to find a reasonable low CP mon with type advantage, then sort A-Z and fill the rest of the slots from a collection of CP 10 weaklings named "a Magikarp", "a Pidgey" etc. If I have to do a lot of runs I temporarily rename the attacker to "1Seadra" or something to select it faster. Usually I train only against the first one and run, only occasionally the 2nd too depends what it is and which CP it has.

I always aim to beat at a minimum 2, usually 3-4 defenders. Sometimes more. It depends on whats hiding in the gym. Like if Gyrados is #5, I am aiming for 5.

I don't aim for 1000 on first pokemon. I find that it lowers my odds of going deep into Gyms. I typically will use Pokemon in 1400-1550 range and fight 2k to 3k range. My typical goal is to win fast as many fights as I can handle and get 2k to 3k per run.

I have my pokedex listed for alphabetical for a lot of my typical prestigers.
*f move name
*e move name
*g move name
*w move name
*b move name

Then before I start I just favorite 6 I am going to use and those are my only favorites so I use them.

I usually do 3 but sometimes it's impossible because people put something horrible like Ditto or something else well below 1000CP and so I ignore it and just start the CP-Adjustment with the second to last Mon.

Mix of 2 and 4.

So what I often do is grab one pokemon with a type advantage, then fill in the rest of my team with Raticates. Otherwise I'll just grab all Raticates and go with that.

closest is 3) I sort by name and have prestiging teams named *1 *4 *7 *8 (for 1.1k, 1.4k, 1.7k and 0.8k CP), all within 50 of the same CP. I look at the first few defenders and decide which are easy to kill and take one of my full team of half the level of those defenders.

If the level gap between defenders is too big, I'll use 2) just focus on one of the defenders and take a team of half it's CP. I don't have to scroll down (which I'd find annoying) since my low CP pokemon are on top of the list

5, I just bubblestrat for XP. Not enough active players on my team in my area to hold gyms, so I just take down the opponents.

Do you find the auto-selection to be useful? It doesn't seem too smart in the order it suggests (eg had Eggs as #1 to battle, when 1st enemy will be Arcanine - granted #2-5 might be Vapes).

Sometimes I take 3-4 half cp mons(3). Then just fill it up(2) For 3k prestige. Enemy analyze wont take any time. You just look at the pokemon and know what to use.
Sometimes I use bubble strat and push the gym on 10.

So what's actually faster? if you have low cp mons as fillers and just keep fighting the 1st one over and over again or going deeper against actually longer (timewise) defenders?

Bubblestrat or karpfest or dittostrat are typically fast because the defender has so few HPs (20 at times).

Prestiging against any Lax/Lapras or Vaps, from my own trial seems to just waste too much time for me.

I choose my strongest pokemons that have a type advantage. Other then that if i can be comfortable, ill do type advantages that have low cp.