How to set value for the multi-select field using netsuite suitescript 2.0 version?

According to NetSuite's documentation, you cannot use this api method to edit or submit select fields - only fields that support inline editing (see SuiteAnswer ID:45158). You may have to load the record with record.load(), modify the values and then submit with record.save().

EDIT: In answer to the updated question, the only thing that appears amiss here is that you are trying to set the values by the display value of the field, where setValue() is expecting the Internal ID of the values. You can either change the values you're populating with the relevant internal IDs, or you could change it to use the setText() method instead:

var strArrayValue = new Array();
        strArrayValue [0] = "A";
        strArrayValue [1] = "B";
        strArrayValue [2] = "C";

   var PORec = record.load({               // Loading Purchase Order Recod

                type:"purchaseorder",
                id:56,
                isDynamic: true

                 });

        PORec.setText('custbody_multiselectfield',strArrayValue ); // Setting Value (Array List) for Multi-Select Fields

        PORec.save();    // Saving Loaded Record

I tested both these approaches and both work for me.