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The answer is really simple.

If you shot at 2x squads of 10 Berzerkers, do not kill 3 models in one squad and 2 in another and then have both Blood Surge towards you.

Shoot everything into one unit and kill 5+ of them per turn. Kill the whole unit all at once.

They cannot Blood Surge if there is nothing left. Two units of 5+ models hitting you will hurt a lot more than 1x 10 man unit.

To be honest a 5 man or 10 man Berzerker unit something still means they die. But if one of those squads only has 1 or 2 models left, that becomes something you can survive and kill.

Also... Overwatch...