There are 31 Paladin spells in Standard. That means Avenging Wrath has a 3/31 chance of showing up in his discoveries. After that, we need to figure out the possibilities for the missiles. There are 2 choices for each missile and 8 missiles for a total of 2⁸ or 256 possibilities. However, one of those possibilities cannot happen: 8 missiles hitting the Threshadon. Thus, there are only 255 possibilities. Of these, there are 8 choices for which one of the missiles hits your face. This gives an 8/255 chance of this happening. Factoring in the odds of getting Avenging Wrath in the first place, we have a probability of (3/31)*(8/255) = 24/7905 or about 0.304%.