Hi Evan,
As far as I know, this cannot be done, if these devices are going offline at the same time, AMP groups them together.
You may possibly try creating multiple triggers in conjunction with different folders.
1 folder per floor which holds a certain amount of APs
Create a trigger for that folder
and so on...
If I am correct and this cannot be done, I would suggest posting this in the Ideas Portal to get it added as a feature request