It sounds both somewhat difficult (the typical galaga player doesn't exactly aim carefully) and extremely tedious, both for players and for spectators. It reminds me a bit of the (possibly apocryphal) story that casinos promote tales of card counters cleaning up because the average attempted card counter doesn't know what he's doing, and will gamble more (and lose more) than someone who knows he's playing a game where the odds are against him.
On the difficulty scale, I'd say it was about 4-5 (on a 1-10 scale, 10=hardest).
Tedious, yes. You wouldn't want to do this trick in arcade primetime (Sat aft-night). As long as there wasn't a crowd waiting to play the game, I felt no remorse in using the trick. I also played mostly on a tabletop version of Galaga so I just sat for a mostly boring 10-15mins instead of having to stand / lean on the game for the same amount of time.
The goal was to get a double-ship by the time you did the trick.
The double-ship made playing the game after the aliens stopped shooting at you much easier since you had double the firepower.
The problem was that the double-ship also doubled your vulnerable attack surface to the remaining aliens.
The remaining aliens would also use attack patterns (diagonal from right to left towards the bottom left corner, almost horizontal, instead of mostly vertical) which increased the chance of crashing into you and ruining your chance at achieving the trick.
You didn't want to kill the two remaining aliens (which would have wasted all the time you spent avoiding the aliens), but you also didn't want to die.
You couldn't take a bathroom break either - there was no safe space to position your ship and avoid the aliens/their shots.
It wasn't a difficult trick to perform. Failed the first few times I tried it, but once you got the hang of avoiding the aliens at that point, success rate was more than 75%.
