i think the xf brakes are similar to e series so get a monkey wrench and the piston depressing tool. put the piston tool on firmly (not too tight) then turn the piston anticlockwise with the monkey wrench. it takes a while, but it does work!!!!!
if you cant be bothered with this, goto auto one - they have a tool for $15 you can use...i guess most auto shop would have then though
if this dont work, the piston may have seized up - you may have to buy another caliper