If that is in the correct drop order, and there is a chance to drop 1 item, than the staff's effective true drop rate is just over 5%. It would be higher if there are more than one drop allowed.
Math if there's only one drop allowed: (1 - 0.1) x (1 - .85) x (1 -.06) x ( .08) = 0.0575)
Keep in mind that you have to NOT get the items above the staff in order to roll for the staff.
If each drop is rolled separately (rather than in-line) than the odds of getting the staff to drop would be two chances at the above chance, which works out to be ~10% (not sure if the same item can be gotten twice).
If they are rolled in-line, that gets a bit more complicated, but the math works out as:
If two in-line items.
Item 1: 10% chance for Leggings, 100% chance to move down list (even if you get item, still have a roll left)
Item 2: 15% chance for Axe, 98.5% chance to move down list (only stop if you get both items above)
Item 3: 6% chance for Maul, 97% chance to move down list (odds of getting 1, or 0 from Item 1,2).
Item 4: So you have a 97% chance to roll for the Staff at 8%.
NOTE: I think items are rolled in-line. Your loot drop is calculated by going down the list.
We could nit-pick the drop rate discussion in far more depth.... but yeah...
There's a small chance to get the staff. Keep killing them till you do.