Hunters are vulnerable during activation to offset the fact that the supers are timed and not instant use. It is one of the ways that the hunter super is balanced. By taking this away or making the others vulnerable, you're actually unbalancing the supers.
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This may actually be true for bladedancer since it has damage reduction after the super is activated. But i dont agree for golden gun. It has the advantage to be a precise range attack with quick shots and has no damage reduction because of that. Thats the way its balanced. But i doubt it would destroy balance if the hunter doesnt loose his entire super when killed in the activate animation.