Regarding the "fake laugh/second laugh" trigger where he goes on to spawn the wave anyway here's what causes that:

When you leave an area (2 screens away) the game "saves" the state of the area as 'active' for about 7 or 8 seconds. Because that area is still considered 'active', it wont re-trigger the laugh if you re enter before that grace period.

For a good idea of when that 'grace period' for active state is, look at some 'seal glitch' videos of people teleing around chaos before hitting the last seal. Monsters in chaos far away from you will drop loot as long as that area was active about 6 or so seconds before the seal pop. If you were too late on popping the seal, some of the chaos will have NOT dropped loot when you teleport back, that happens because those tiles were considered 'inactive' while you popped the last seal. Llama also explained more about this 'grace period' in his newest TP Glitch video, though that tp trick has little to do with this glitch. here:

We're trying to time baals wave spawn/throw animation and us leaving the throne room area/line within that grace period time (7 or so seconds maybe?), and entering the throne baal area for 1 or a few frames of time before you get into the 'inactive' territory of the area. If you get the 'fake laugh' it just means you were too late back into that area and it became inactive. This can be caused by you toeing that range line short and thus were not considered in the area within 7 or so seconds by the game. This is also why it's important you leave baal's range area at or near the end of his laugh, to give you enough time that the game still counts it as within the 'grace period' when you do the trick to step back in throne 4-6 seconds later and baal is ready to throw the wave.

Walk method is easiest because you have precise control. (success confirmed) Tele method you need to stay in baals range and time it with the exact frame(s) before baal throws his wave to leave the range. (Success confirmed, by teo's vod and I've done it as well) Theory Town Portal Method: You open a town portal over 1.1 screens away from baal (do it 1.5 screens or so away to be safe) and time going to town the exact frame(s) before baal throws his wave - thus leaving his 'range'. Your town portal wont be in range because it is more than 1.1~ screens away and has a smaller range than your charecter. (have not confirmed this method)

Mercs position should not matter so long as youre leaving the range before the wave spawns.

Heres a couple of videos on the range: video 1: https://streamable.com/whk9gq video 2: https://streamable.com/p3kiyl Notice the minimap monsters, on the right/lower side theres a point where i'm past the 'line on the floor' but still in range of monsters. It seems to be better to try it near the middle or the top of the area than the bottom area because of this 9-11 seconds in the second video is where this occurs and is easiest to spot

Process to skip a wave:

1. Make baal laugh, wait till near end of his laugh, leave range of baal, wait about 4-6 seconds
2. Enter range again for 1 or so frame(s) (baal will decide to spawn as soon as youre in range), LEAVE INSTANTLY
3. Game will think baal spawned wave (but wave doesn't spawn if you were out of range in time),
4. re-enter range- if he laughs again you know you skipped a wave, if not you know the wave has spawned and you were too late leaving the range.

Repeat. Can be done on all waves. The hardest part is the timing in step 2 to leave instantly before the wave spawns.

I've done this successfully a about 10 or so times total so far. Can be done with teleport but walking seems much more accurate, teleport is just too slow and has too many frames.

If you use a maphack with show monsters, you can see the monsters despawn if you're 2 screens away. The limit for throne room is just before you exit the actual room. I think baal might be doing the animation to throw the new wave, but you are leaving the throne room JUST as baal is about to drop the actual wave, he's mid animation AS you leave the room, so the monsters dont actually spawn. When you re enter the throne room and baals range,it registers as a "done wave" baal laughs. so yeah you probably have to be SUPER accurate on the timing, and it can only be done when 1 player is in the throne room area and no tp's down by baal area so the monsters dont have a chance to spawn while baal drops the animation.

Notice where this amazon is standing in relation to throne room, use the two fire pits for an idea of distance: Here is where the range of the wave is visible: https://imgur.com/YazXL61 Here is where the range of the wave is no longer visible: https://imgur.com/WLL0N6h

Just tested some, seems like the game decides to spawn the wave either AT the first frame of baals animation or before the first frame of baals wave spawn animation. I would tele out of throne as soon as i predicted his movement and the wave still showed up, so it's probably just a super tight window where theres a couple of frames of room for something like that to have a chance of happening.