If I hold on a battlefield but my opponent has a Titania Crownguard and I kill Titania during my turn, can I score on the battlefield where I held?

No. When you attempted to Hold that battlefield at the start of your turn, the battlefield was marked as "Scored" for the turn — even though Tianna prevented you from gaining the point. You can only Score each battlefield once per turn, regardless of whether a point was actually gained (Rule 465).

Killing Tianna later in the turn doesn't undo that scoring. If you leave and re-enter that battlefield, you won't be able to Conquer it because you already Scored it this turn. Your only path to victory in this scenario is to Score the other battlefield that turn (as detailed in FAQ 9615/9179).