Your grandfather's unit, 259th FA BN, was activated at Camp Swift, TX, 25 Jan 43; following training, it departed NY P/E, 11 Aug 44; arrived England, 22 Aug 44; arrived France-ETO, 16 Sep 44; arrived NY P/E, 9 Mar 46; inactivated Camp Kilmer, NJ, 10 Mar 46.
Ardennes-Alsace (Battle of the Bulge) Central Europe Northern France Rhineland.
This unit was non-divisional, meaning that it was never a permanent part of a division, corp or army, but was assigned unit-to-unit an an as needed/where needed basis.
I have no information as to the unit's casualties.
I have reviewed all Infantry Divisions, and could not find the 259th FA BN as having been assigned to either one. Therefore, they would have been attached at the Corps or Army level, I would assumne.
I am sorry that this is all of the informatin I could glean for you.