What is the interaction between Wampa and shielded targets?

Although the timing is a little bit in question, you do exactly what the card says. You roll your attack dice, if you roll at least 1 crit you can cancel all your dice and if you do that you deal a single face down damage card to the defender.

His ability doesn't really interact with shields at all.

Dealing a damage card completely bypasses shields.

See RRG page 9, Damage, bullet point six.

See RRG page 9, Damage, bullet point six.

Thanks for citation. Impossible to know who to believe otherwise!