>>35148839>Illusion no longer breaks on taking a hit that would have had the same effectiveness on either Dark, or the pokemon Zoroark is copyingFor example. let's say Zoroark is impersonating a Hitmonchan, and gets hit with Ice Beam. It's neutral on Zoroark, and neutral on Hitmonchan, so Illusion doesn't break.
For example, Zoroark is impersonating Serperior, and is hit with U-Turn. Both Zoroark and Serperior are weak to Bug, so the Illusion holds.