text: You're not going to make much headway with arguments like that.
  
  What you should do is to undercut the entire idea of sacrifice as the essence and model of perfect love, but you won't be able to do that using Scripture!
  
  The notion of offering - a priest offering for others - goes too far back in Scripture for you to undercut it.  Remember Abraham - carrying wood on his shoulder (a priest's role!) - for the sacrifice?
  
  So Jesus raised & perfected the image of sacrifice and the priest.  Priest & victim as the Catholic Church teaches. 
  
  And you'll have to argue that "died once and for all" should be understood in purely human terms, temporal, once & for all, done.  Rather than "once & forever more".
  
  Catholic priests "partake" in the ministerial/liturgical role of Jesus's ONGOING salvific action; we "partake" in the "common priesthood" role, by offering our own work, struggles, efforts, sacrifices TO GOD in union with the ongoing sacrifice of Jesus in the Mass. One sacrifice; unity.