Risada, how are you getting an attack roll of 1d20+10?
You are normally at +9, then you took a -2 for Power Attack, which would put you at +7. Were you attempting to factor in the -4 the tanarukk takes to his AC for being prone?
I'm a dunce. You have a bonus from Knowledge Devotion...
Edit 2: but that's only a +2, which would get you back up to +9. Where's the extra +1 coming from?
I don't think it much matters. His AC is 15, then -4 for prone = 11, so you hit even without whatever the extra +1 is coming from.
Edit: don't forget that you get an iterative attack; you can attack the healthier of the two as well.