I’ve been to enough ACs and from my experience, numerical reasoning tests done on the day are no harder than the online ones. If anything, they’re actually easier as they know you’ll be a bit nervous on the day and they will also put you thru various other assignments (eg group exercise, in-tray, presentation, written exercise etc) designed to assess your other competencies so will lower the pass mark a bit. The numerical reasoning test is merely done under timed conditions to make sure your score on the day isn’t too far off from your online one. I don’t know which company’s AC you’ll be attending, but say with PwC, I found their numerical test at AC (done on paper) to be a lot easier than the online one. There were 20 questions in 20 mins and I think I did 17-18 and got them all right without even much practising! I heard one candidate passed their PwC AC (in which you have to pass all parts of the AC: logical/numerical tests, group exercise and written exercise) by doing only 10 questions out of 20 in numerical and getting them all right.
But if you really want advice as to the best way to prepare, just practise more tests from Assessmentday. There’s another fine practice test provider called JobTestPrep for a bit of variety. The more you practise, the faster you can read and understand the data and the more likely that you can discover shortcut methods (eg for % change from 2900 to 3200, just key in 32/29 then read off all the digits after the decimal to get 10.3%). These shortcuts can save you precious seconds!