Well, many raid bosses are pretty stationary, and its against those that we have the most problem with lag.
Trigger the grid after Harry 5, Horoth or Nyth haven't moved for 20 seconds.
Keep a grid on Velah/Harry 4 since they don't move. Coding that in though might be a nightmare, but it would significantly decrease calculations from checking collisions to simply comparing radii with weapon range.
Or even just set up certain areas for certain weapons around him, and check to see if the players are in that area. That's just a simply statement. The cycling through each area might be inefficient though...
I can dream, can't I?