Screw head of Chuck stripped

[Giggitygoebbels]

My Homelite 480W hammer drill have a bad keyed chuck,and i want to replace it.I used a drill driver using a phillips 2 head in clockwise direction.However the screw is so tight it only partially shed the head of the screw off.How can i remove the screw without doing more damage?
The torque is at 5 for the drill driver.Even when pushed hard to screw and using a traditional screwdriver it just damage the head further.
Thanks in advance.

 

[pinenot]

This is the holding screw on the inside base of the chuck your talking about?
Drill of the screw head, fit a large pair of mole grips or a pipe wrench on the chucks outer barrel tight, then smack the extended handle sharply with a mallet or similar, anti clockwise. This should break the seating of the chuck > shaft threads and allow you to remove it...pinenot

 

[foxhole]

My Homelite 480W hammer drill have a bad keyed chuck,and i want to replace it.I used a drill driver using a phillips 2 head in clockwise direction.However the screw is so tight it only partially shed the head of the screw off.How can i remove the screw without doing more damage?
The torque is at 5 for the drill driver.Even when pushed hard to screw and using a traditional screwdriver it just damage the head further.
Thanks in advance.

Screw holding chuck normally a left handed thread , which would explain why it snapped screw head.