A question for would be judges (but I think I know the answer). Not strictly a Duels question but I guess it may be relevant in Duels matches as well.

Scenario is player has Mindclaw Shaman in hand and casts it. His opponent has Scatter to the Winds. Can the opponent legitimately target Scatter to the winds given there is no legal target spell to cast it at?

I guess I am trying to determine and check that Mindclaw Shaman cannot be used to cast any instant or sorcery unless a valid target exists for it.

You are correct - if there is no legal target you can not use that spell with Mindclaw Shaman's ability.

'If you can’t cast any instant or sorcery card (perhaps because there are no legal targets available) or if you choose not to cast one, then Mindclaw Shaman’s ability finishes resolving and the game moves on'.

Thanks Sjokwaave. Its as I thought. The spell has to have a valid cast outlet otherwise it can't be cast.