(function(){

var alias1 = HitCalculator.calculateAvoid;
HitCalculator.calculateAvoid = function(active, passive, weapon, totalStatus) {
var avo = alias1.call(this, active, passive, weapon, totalStatus);

var turnType = root.getCurrentSession().getTurnType();

if(turnType == passive.getUnitType()){
if( SkillControl.getPossessionCustomSkill(passive,'seiryu')){

avo += 20;
}}

return avo;
}
})();

こんな感じでどうでしょう？