If you're interested in actually doing this achievement legitimately (for the 1% of you who are), the most important aspect of this is the most obvious: practice.
First and foremost, you need to become proficient at expert level. You're going to have to play a good amount of medium and hard to get there, and then an even larger amount of expert until you can consistently get 5 stars on most expert songs. No way around it.
Unfortunately, you're about halfway done at this point. Now it's time to introduce a second pedal and begin expert+ mode. It will feel very awkward at first to use both feet if you've never done it before. But with time, you'll get the hang of keeping a steady, pounding rhythm by alternating feet. A great song to get good at in order to build these skills is Laid To Rest- it's not nearly as hard as Raining Blood, and it builds your leg proficiency.
Once you've got the alternating kick patterns down it's just a matter of increasing them up to the speed you need for Raining Blood. This is no easy feat, so it will take quite a bit of sticking with it to get to where you need to be. Once you finally decide to tackle Raining Blood, you may find that you can do decently in the beginning of the song, but you tire out after a minute or so. In this case you may want to pause the song quickly and take a few minutes to recharge, before you continue to the end. In the long run, this will help your score far more than it will hurt it.
In the end, this is quite possibly one of the hardest drumming achievements in any GH or RB game (along with a few from Green Day RB), and it's a female dog to get. If you do decide to follow my advice, you will not only pick up the achievement in the process but a newfound love for the series as well. Heck, I got it months ago and I don't see myself quitting GH any time soon.
Peace, and good luck.