You can go underneath the platform to the right of the pod the boss is released from. You can still take damage but I found it easier under there. I was only doing the heroic at the time as my warlock isn't high enough for the nightfall.
The same strat. should apply for the nightfall. The priest stayed around the upper area while I was under the platform.
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The only thing about under there is if your team is all under there and you die, you'll get revived outside the platform and probably be killed right away
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Didn't realise that - but a non-issue if ur solo as the ramp provides complete cover to heal up