How to get A rank in Onslaught?

The mission requires you to get an A rank at the end of Onslaught which... is VERY difficult and I think the point margin for it is 4000 points. However, trying to beat some of those stages in such short times (50 seconds at lvl 8 hah!) is ridiculous and then you have the added difficulty of making sure no-one dies for the points. I really don't know how to get the points necessary to win it (Beating it is difficult enough) and apparently, there is an even harder, premade party one which requires you to get S rank... I'll pass on that one thanks...
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