It seems like the blocking is auto. There seems to be essentially a timer. When you first enter as the hero, you can see the life bar on the bottom depleting by 1 seemingly every second, maybe two. If you get into a crowded area where you're being shot by multiple people (or having grenades chucked at you), that bar drains that much faster.
Pretty much similar to when running around on the map as a regular rebel or stormtrooper, you don't want to go running into a mass of enemies because it'll just get you killed quicker. Find the quieter route or a flank and try and get them in a groups of 1-3 and decimate them that way.
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