Board Thread:Questions and Answers/@comment-28154750-20160407232430/@comment-25004901-20160407233642

You can declare final phase whenever you want, actually, you do so no matter what since you can't skip phases, you just can't cast impact cards. Now, on to the "when I can kill them" part, well, as long as you meet the requirements, you can cast an impact card, but only in the final phase UNLESS it has counter. For Impact Monsters, it's basically the same thing as impact cards and when you can or cannot cast/use them. The reason why most hold out on casting them, is because it's usually not best to cast certain impact cards unless you can actually kill them.