Can Viktor's signature spell be activated to buff units when an opponent plays Volibear's signature spell targeting units at a battlefield?

Ruling: Action cards can only be played on an opponent's turn during a showdown when no chain exists. Since the opponent has played a spell, a chain exists and only reaction spells can be played at that moment.

Nuances:

• The card being referred to (Grand Strategem) is not Viktor's signature spell, it's an action card
• Viktor's actual signature spell (Siphon Power) is a reaction and could be played in this situation, but only gives +1 might, not +5