It is possible to survive a charge by 3 Onus and still win the game. Doesnt happen often though
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Happened to me -- We had a standard outpost, marines stretched thin between Lava Falls, and Repair... can't remember the map name. Anyway, they three oni came around the corner and charged at what was 3-4 marines including me. This proved to be overconfident because as soon as they did, our commander beaconed us all in and since they were in between the mishmash of buildings and stuff it was total confusion for them as one was killed almost instantly, and another while trying to escape. After that, the game turned completely around. I think it was the shotguns.
It seems like either you win at that point, or you not only waste your chance, but you also turn the tide in the enemies' favor.