Has anyone tried this technique for keeping the opposing team from using golden rasperiies?

I haven't gotten error 29 when leaving mid fight with a defender. I've gotten it often when knocking out a defender and then running away prior to fighting the next defender. For this method you aren't backing out between defenders just during mid-battle. Seems like it works.
I've done something similar with another Instinct player with two high level accounts, but we stagger attacks. Took a fully motivated gym down in less than 5 mins. He goes in first and once he gets halfway through the 3rd defender, I go in against the 2/3 motivated squad, then he goes in with his third account a little while later. It isn't fool proof, but is very unlikely that someone who isn't botting would be able to golden razz in time, unless they're sitting at the gym waiting for a raid.

I have done it with two players against a single defender only using golden razzberries at 1/3 motivation. If more players are berrying, and at higher motivation, you need at least three players.

Here is the original Reddit thread:
https://www.reddit.com/r/TheSilphRoad/comments/74glq3/the_art_of_synchronization_an_ultimate_teamwork/

That article is using the initial assumption of joining after an enemy's HP is lowered past some threshold. The updated requirement is joining a fight, then leaving and rejoining to start a new instance.

- Everyone starts together.
- At least one player stays while others leave and restart to create new instance of the fight with the same defender.
- If motivation is at 2/3 or higher, then you need to repeat to create a third instance.
- Everyone finishes their separate instances at as close to the same time as possible. More time in between finishing instances provides more time for the defending team to use golden razzberries. If everyone finishes within a second or so, there is no time for golden razzberries.
- When you knock out the first defender, the 10 minute timer starts. You have to repeat this for the next five defenders within the 10 minute window or the defending team can add in new defenders.
- Whenever a defender is knocked out, be sure to move on to the next fight before anyone leaves to minimize the chance of the attackers getting locked out.

Have used it successfully.

The error doesn't affect it, no player needs finish a mon without moving to the next one.

It works as long as no stray attackers enter the fray.