This riff is pretty straightfoward. We'll be using all downstrokes as well.
The picking in bars 19 and 20 might be a little tricky at first. We're basically switching a bass note melody from the 9th fret A string(F#) to the 7th fret Low E string(B), all while inserting two open D string notes in between each bass note. Practice bars 19 and 20 separately to get used to the pattern.
Lastly, we have 2 natural harmonics in the final measure. Place your index finger gently over the 5th fret of the A and D strings, then pick them both. You should get a nice natural harmonic sound.