AAA means that you need to have a very high percentage of steps Marvelous and Perfect and very few Greats, nothing below that. Full combo with precision, basically.
I got this on the Devo song (Watch Us Work It ) with 96 Marvelous, 35 Perfect, 3 Greats on Basic.
Hold down A when you select a song to bring up the options and tweak the settings - particularly the speed multiplier - to what helps you nail the steps. I play better when it is faster, so I generally use 1.5x and Boost or 2x.
It also may help to turn off the announcer, lyrics, and dancers as well as turn the background brightness way down in the options. This provides less distractions while playing, and can help focus on the steps.