Does Arachnidar Claws and Ultimate Blast work together?

Yes, if you use Arachnidar Claw's Ultimate Attack (+8) you will be down to base armor, and then if you play Ultimate Blast (+5) you would then blow off your base armor head. You would lose your head but you would be rolling at least +13 dice in your attack.

Note: If you reverse the order and play Ultimate Blast first, Arachnidar and all stacked head armor would be discarded and you would only get the bonus of +5 dice.