A while back, I thought I was going to have similar results storming a castle with a Sphinx inside. I also did not wait for the dominion to lower, so was faced with a 1300+ hp Sphinx at the time.
Much to my surprise, once my vine ogres broke inside the castle and surrounded it, they easily beat it down within 10-15 rounds or so (could have been more, but they seemed to be doing 100+ damage per round).
I am not sure whether Astral Shield was up, but I would assume so. I see vine ogres are mindless. Does that make them immune to the paralyzing effect of Astral Shield? Perhaps I was just lucky, but I seem to recall the vine ogres killed it alone without any debuffing at all
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