The only way to overwrite a galvanize by a SCH is to cast a stronger one. Either galva or succor.
Only the AST can effectively remove it with his spell (forced overwrite without looking the amount of shield)

Since you were both ilevel synch up the only factor was crit

In 8man if the other heal you can just go DPS and act as sub-heal in time of crisis
Otherwise kill some thing X3