WoWDB

To create the charged  Charged Chitinous Focus, you will need to be on the upper level of Beth'tilac's room and place the item shortly before she performs her  Smoldering Devastation. Then head back up to grab the result.

Note: Even on 25 Heroic, it is quite possible for a competent group of level 90s to DPS the spider boss so fast that she only gets off one Smoldering Devastation. If your group is not willing to slow down on DPS, then just loot the  Obsidian-Flecked Chitin Fragment and wait until the next lockout to charge this item.

Also IMPORTANT NOTE:  I discovered today that it's possible to use the focus outside of Firelands.  /cry