There is no way to shred on an acoustic guitar easily. Acoustic guitars can be shredded in two ways. One is with a pick, and the other is with your fingers. Despite its ease of learning, picking is also one of the least satisfying methods. Picks are great for getting that classic 80s shred sound. But if you want some high speed and aggression, here is how to play fast with your fingers:
Practice slow before going fast. Slow down all of your songs, so they are at 120 BPM or less (or whatever speed you are comfortable playing at). You can focus on hitting each note cleanly without worrying about getting lost in the song's tempo. Practice slowly until you can play everything perfectly at 120 BPM or less.
Once you have that down pat, start increasing your speed slowly until it becomes too hard for you to play accurately at any given tempo. It should be somewhere between 160 and 200 BPM depending on who's doing the shredding, then drop back down to 120 BPM again, and practice until everything starts coming together much better than before.