Given my lack of skill with the drums on rock band 1 & 2, I decided to try an alternative strategy for getting the drum trainer achievements that actually worked out quite well on both beats and fills. The achievements for the drum trainer are stackable, so I started at 180 BPM for both challenge types and simply had a buddy of mine help me out with the ones I couldn't handle myself. In some cases, he would pull the bass pedal aside and take care of those parts, leaving me to focus solely on the pad notes. If I had to do pad notes with off beats, I'd focus on the steady ones (usually yellow or blue) and the bass part, and let my buddy hit the pad for the other color notes (usually red). Got all 4 achievements in about 5 minutes.
Hope this helps!