You're talking about the sinspawn right? Best way to kill it is to lop off its arms, which should take 2 hits with Auron, and then kill its head. Then summon Aeons to take care of the body, destroying the arms when they re appear.
And the items option will be further down on the menu, you can scroll down the little box. It should be there. You can have more than three commands now, and as you get new ones, the older ones get moved down.
