Shark Shredder, Killer Clone

Spells can only be cast for their sneak costs any time you could play an instant during the declare blockers step on your turn (after your opponent has decided whether to block).

In the case where Shark Shredder deals combat damage to a player during the regular combat damage step (probably because another effect granted Shark Shredder double strike), the creature returned with Shark Shredder's last ability won't deal combat damage at all during that combat phase.

If Shark Shredder deals combat damage to a player during the first strike combat damage step and its last ability returns a creature with first strike, the returned creature will still deal combat damage during the regular combat damage step. If the returned creature has double strike, it will deal combat damage only once during that regular combat damage step.