Hi

just had a game on OCTGN and my opponent tried to use nullify when I Used an event card to shield a unit but i argued that shield cards are not played but discarded from the hard.

my opponent graciously conceded the point but I would like to clarify if nullify can be used to cancel event cards used as shields?

From Events in the Rules and References:

When a player plays an event card, its cost is paid, its ability is resolved, and the card is then placed in that player’s discard pile.

From Shields

When a unit is attacked or dealt damage by an effect

its controller may discard one card with one or more printed shield icons from his hand to prevent an amount of damage up to the number of shield icons on the discarded card. This is known as a shielding effect.

Based on those two entries (and years of playing LCGs), I would say you are correct: discarding for shielding is not playing an event.

Also, the wording of No Mercy:

Interrupt: When your opponent uses a shield card ...

Correct.