Its pretty easy to make Shield grant immunity to magic missles. You just need to add the opcode Protection from Spell [can't remember right now], and then use the resource of the magic missle spell. Since there are commonly several versions of the spell, it is important to make sure you have gotten all the relevant ones. For example, from memory, in BG II magic missle includes SPWI112, SPWI003, BALTH06, and one other. To grant immunity, you just add these resources to the protection from spell. Shadowkeeper's spell browser, or near infinity are good ways to look for references to Magic Missle, or any other spell to see if you have gotten them all.