It is important to note that the timer begins as soon as you click the door from the gargoyle/bell event. Do not click the door until you are ready to kill 20 bats in 5 minutes.
The random swap has an emote paired with it which reads "You vision blurs momentarily, and you find yourself somewhere new." This is not a typo - the line is "You vision blurs" not "Your vision blurs". If you are boxing, or even if you want to prevent yourself from being disoriented, this is the string to use in GINA or the in-game audio trigger system. It is important to be aware when you get moved around since you generally won't want to be anywhere near floor 2 unless you're a ROG/BRD disarming traps.
Each wave has different numbers needed for the bats.
Wave 1: kill 20 bats on the top floor (B1) in 5 minutes
Wave 2: kill 15 bats on B1 and disarm 20 traps on the next floor down (B2)
Wave 3: kill 10 bats on B1, disarm 20 traps on B2, kill 10 zombies on B3
Wave 4: 10 bats on B1, 20 traps on B2, 10 zombies on B3. Be aware that the orcs in B4 hit fairly hard and are tethered to the room, they cannot be kited in the bell room.
This means that B1 becomes easier as time goes on. Note that on each floor, the thing being spawned spawns wherever the NPC is standing - the bats spawn on top of the truefaith batkeeper, the traps spawn on top of the Truefaith sneak, etc. If you're trying to low-man this, or even if your raid is undergeared and you're trying to optimize, your best bet is to park your entire raid on B1, kill 20 bats, then send ROG/BRD to B2 to prepare to scan for and disarm traps. Watch for the emote to know where the trap is and don't go near it until you detect/disarm it. Leave the rest of the raid on B1 to kill 15 more bats.
Once that is done, leave enough folks behind to kill two bats every sixty seconds. The rest of the raid should jump from B1 to the ramp leading to B3. Do not walk through B2 as you might trigger a trap by mistake, and triggered traps cannot be disarmed to count towards the needed 20. The rest of the raid on B3 will kill the zombies. After wave 3, send a heal/kite team down to B4 and cross your fingers nobody crucial gets swapped.
If you have any MAG, they're best off on floor 1 since the bat team getting swapped to anywhere else will make it difficult for them to get back up to B1.
On each wave, when you have reached the amount needed for bats/zombies killed, no more will spawn until the next wave begins (all four floors share the same timer). I can't confirm whether or not traps continue to be set after the 20th is disarmed.
Also note that you don't seem to be able to /rewind back to your original position after being swapped.
Once the event is complete target and kill the bell on B4 and a trap door is revealed beneath it. This can be clicked to go to event #3, Unwelcome Guests. I do not believe any timers start upon clicking in.
Edited, Jun 27th 2022 10:27pm by Andhrimnir