"It's Time to Say Goodbye - Leaving Punk Hazard!" is the 623rd episode of the One Piece anime.

What episode does the doflamingo arc end?

Episode 744 is the episode that you're talking about. The filler episodes is 747-751 then it's the Zou arc.

Can I skip Z's ambition arc?

The Z's Ambition Arc is the first One Piece arc to occur after the time skip. The episodes also act as a set-up for the film, One Piece Film: Z.