Me and another mage had an argument the other day, where he was saying Arcane Focus reduces the chance for mob to resist your spell with his "resistance" rating, and i was saying it works just like +hit%.

Does anyone have a confirmation on this?

Yes, AF is really +hit. It was confirmed by a blue post a long while ago, though I can't find that post anymore. I *think* the original Elemental Precision was worded the same way (but was 6% total in those days and no mana reduction) and that caused Tseric to clarify, if you're trying to track that post down.