They definitely can be killed during Fist of Havok. It has happened to me lots when I was low on health when I popped it. Though if the titan is full health, with the proper upgrade then they are nearly impossible to take out once they activate FoH.
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I've played this game since release and still haven't killed a titan during FoH yet.
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Run into me sometime. That's when it'll happen.
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I've done it before, Gjally to the face as he jumped at my team. When the smoke cleared, they were all still alive, so I guess it worked. :)