The Z ability was very difficult to use on the Assassin Shadow Hunter. Unlike the Deathblade the Z bar took longer to fill, and then after hhitting max fill, it reduced fill by 25% after 15-30 seconds, zoning, or getting on a horse. The timing out of all my playtime allowed use on 2 boss fights, and one was just a middle area boss not the end dungeon boss. You could not plan to hold the Z ability for need, it was use it or immediately lose it, so it got mostly ignored.