You must cast Dragon Grip at the latest during the declare blockers step after blockers are declared if you want first strike to matter during that combat.

If you're using Dragon Grip's ferocious ability to cast it as though it had flash, you must control a creature with power 4 or greater only as you begin the casting process and put Dragon Grip on the stack. Losing control of that creature while casting Dragon Grip (perhaps because you sacrificed it to activate a mana ability), or in response to Dragon Grip won't affect Dragon Grip on the stack.