Much easier on the easier difficulties as you dont have to worry about enemies killing you. I did it on hard though as I didn't feel starting a new game was worth the time. Its not that bad on hard if you're going back to do it after beating the game, which I recommend doing if you want a more efficient scab-killing character.
I did the entire first act levels and the first 3 in the second. I also did pretty much every time trial level as they have the most condensed scab plants. Some might be a little tricky. I would avoid doing the ones where you have to kill enemies to open the green walls, and the ones with a rediculous amount of scab plants scattered throughout the level such as the last level.
Get good with hologram riding as its usually best to be in the air while you do some levels so you can shoot over fences and already be at a high elevation to get hard to reach scab plants after chaining one from the ground.
Always run over the scab plants whenever you can for powerups and just keep picking them up. Regenerate and barkskin can really help you ignore enemies and just go for scabs. Be careful when ignoring enemies though as framerate drops dramatically and my game froze on a few occasions.
Can't really think of anything else. Besides the first levels and time trials test what levels are easiest for you. I tried every one out and did the ones I knew I could do with just a little practice.