Problems getting Grenth's Balance

I have gone on a total of four runs on the Facet of Darkness, and not ONCE has it used Grenth's Balance. I have even tried spamming signet on it, to no effect. I tell the group to go easy on the facet, but after about 4-5 minutes they get bored and kill it. Any suggestions to get the skill?

Drop it below 50% in health, with someone at ~100% health near it. It should use the skill. If you miss the skill, do the same again, remembering the cooldown on the skill.

Yeah, it only uses the skill when it is low on health. Get people to kill it slowly, and it should use it before it dies.

Thank you, I now have the skill

BTW i used it on Glint and hit her/him(what is it???) for 588