Damage to Shield first

var list, col, ins, hit;

hit=false
list=ds_list_create();//create list
col=instance_place_list(x,y,all,list,true);//list all collissions


for(i = 0; i<col; i+=1){
    ins = ds_list_find_value(list,i);

    if(variable_instance_exists(ins,"game_health") and ins != id){//check if instance has health
        if(variable_instance_exists(ins,"game_shield")){//check if instance has shield
            if(ins.game_shield>argument0){//check if shield is above 0
                ins.game_shield-=argument0//damage shield
                hit=true
            }else{//if shield = 0 or below
                ins.game_health-=argument0//damage health
                hit=true
            }
        }else{
            ins.game_health-=argument0//if no shield, damage health
            hit=true
        }
    }
}

ds_list_destroy(list)//destroy list

return(hit)//return if an instance has been "damaged"