First, don't use giga impact. Swap out dark pulse for a stronger move in crunch, and give Tyranitar a coverage move like fire punch, or a boosting move like d-dance.
As for the item, Assault vest/leftovers/smooth rock should work well.
You could also go for Tyranitarite but its better to just run a normal Tyranitar and save a mega slot (Gen 6-7 only)
If this were for a playthrough, I would recommend just simple boosting items such as soft sand, black glasses, etc.
(Yes I included Tyranitarite since I read the comments)