I concur with
@
TheAverageFan
that the defense reduction part of the ability is wordy and difficult to parse at first glance.
As it reads, the Special Attack has a roughly 72% chance of triggering the defense reduction every time you use it. The only way I can think of to game it is by using that attack on OM3 on a round where you lost initiative so that you have a chance of taking the initiative and revealing OM1 before your opponent reveals an OM.
I think it would be both more interesting and easier to understand if the condition for the defense reduction was changed. My initial thought is to have it trigger when Bestiarius is wounded by one of the excess shields. The wording being something along the lines of:
"
PRECARIOUS SWING SPECIAL ATTACK
Range 1. Attack 7.
When attacking with Precarious Swing Special Attack, all excess shields rolled by the defending figure count as unblockable hits inflicted by the defending figure on Bestiarius.
If Bestiarius receives a wound this way, subtract 1 from his Defense until the next time you reveal an Order Marker on any Army Card."
That way, the probability of the ability triggering is dependent on the defense of the figure you're attacking rather than being static. If you wanted to make it trigger more frequently, then you could change the wording to "If Bestiarius did not wound/destroy a figure with this attack... (defense reduction)"