CLARIFICATION: I found that the achievement description left me with questions, to basically sum it up "If I completed 24 songs on easy, getting 5 stars, and I couldn't get another song on easy would I be screwed?" The answer is no. I know this because (for the Pro Guitar achievement) I got 3 stars on 24 songs on medium, and I suck a Low Rider so I got 5 stars on easy.
This achievement description should read "Complete 25 different Rock Band 3 songs on Pro Bass, getting 5 Stars on Easy or 3 Stars on a higher difficulty."
To get this achievement I recommend playing on medium as it is much more forgiving (star and score wise) and as a result easier to get the achievement.
Playing the easier songs will help too so I advise that you sort by difficulty and proceed down the list. If a particular song gives you trouble it might be better to skip it and move on.
However if that doesn't help and you feel like you've hit a wall skill wise, Rock Band 3 has a new mode in the "Training" Menu called "Learn A Song". This will allow you to play sections of the song you wish to improve at much like you would in the instrument trainer.
One tip for playing that I have is to utilize the pause button. This will help you position your fingers properly so that you can hit a challenging note or chord, hopefully without breaking your multiplier. USE WITH CAUTION, If you pause too many times during any one song the game will sense foul play. You will know because your star meter in the top right will be reduced to zero and will not progress.
I hope my guide helps.