A glitched acolyte in the Rocketyard on Plaguelands and he was completely immune to damage. Fired 7 Gally rockets and full stormtrance and didn't even scratch him.
Randall hath been dethroned as the most difficult enemy to kill in Destiny...R.I.P. :(
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Me and some friends had a splicer shank in the forge that wouldn't take damage. I hit him with a melee and made him a friendly.