I've done the triumph „rabid relic“ in vault of glass 3 times now.
We killed all praetorians only with relic-super-damage but I didn't get the triumph.
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Something I remember being a thing: if you're using stasis to freeze-lock the Praetorians, you need to be extra sure that they die to the shield, not stasis damage. The relic super apparently has a tendancy to partially whiff, so what can happen is you shoot the praetorian while frozen, he "survives" the super shot, but dies to a stasis explosion, which is a wipe(for rabid relic). To avoid that, make sure there's no stasis effects on him when you're ready to super on him. Also, you can mitigate the chances of the super whiffing, if you smack his shield off with the relic melee, and then super him.