What goes wrong? Are you accurate but slow, or do you strike out?
If you strike out, you should revisit each puzzle you failed (click on the red symbol for that puzzle) and do it in standard puzzle mode, making sure to get it right if you possibly can. This will help build up your repertoire of patterns, which is really what Puzzle Rush tests.
If you are slow, just keep at it and you are likely to speed up.
I need help in puzzle rush got any tips?