not professional but... i guess best shield is northern star(or something like that) it has +1 all guru skills / +armor / +160-260 luck as for best shield attack there is only one and that is shield turn, i guess for PvM u want maxedd shield turn & shield master. as for auras its up to you, if you like high defense you go fo aura of defense, or if you like life regen you go for aura of renewall. you also want to max grand aura for bigger "holy aura"(white thgin around your guardian). you also want some points invested on prayer of healing.
so.. skill setup would look something like this..
http://hellgate.ingame.de/charplanner/guardian/?l=50&aa=0&a=00000000&s=021000050a070000002007000500the rest 10 skillpoints are up to you to use. as i said, just choose the aura of your choise. most used ones are defense and renewall i guess.