Scenario: I attack with one creature. Opponent blocks with one creature. Can I then cast Shock on the blocking creature?
Yes, you can; Shock is an instant so you can cast it during the Combat Phase, in the Declare Blockers Step in your example.
Just notice that even if Shock is enough to kill the blocker, your attacker will stay blocked.
...and then the opponent can decide how to handle that damage (ie take the hit or assign a creature to take it).
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Thank you. The combat mechanics are clearer now ;:-)
The Basic rulebook, read it! A lot of basic questions are answered there!
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