How can i improve My Warrior Sunder Damage ?

Spec out of resolute technique and get some accuracy if you need. Losing sunder autocrits is not worth the tradeoff.

Get a better mace, that mace is really bad for physical slams. Aim for something with +physical damage and maybe melee gem level.

Respec the three nodes up to Resolute Technique and one fire node in the wheel at the top right.

Spec these 4 nodes up to Crushing Verdict in the bottom left.

Pretty sure that should be a noticeable dmg increase.