This is more of a generalized solution.
Your best bet is to go for this on guitar as guitar will have 3 challenges instead of 2 on bass, drums or vocals.
The idea is to get 2 (or more) diamond challenges at the same time in one single attempt of the song.
It does not matter if you have already obtained the Diamond challenge before, you can still track progress on challenges you have already completed and they will still count towards the achievement but you'll have to achieve them again on your attempt.
You may have to play on higher difficulties to get this as Diamond challenges tend to be tailored to only be obtainable on Expert and you will just fall short on Hard difficulty in some cases.
You may be able to get 1 diamond challenge on Easy/Medium but this trophy requires you to do 2 in one go.
Best way to tackle this if you are unfamiliar with Hard/Expert is to press orange on the song selection screen and select Guitar Intensity. This will place the easier songs at the top and the toughest songs at the bottom.
The easier songs on Hard/Expert will be the best way to get this trophy anyway, the challenges on these songs will be the easiest.