I finished painting my first large-scale tyranid a couple weeks ago. Here's some pictures and random thoughts on painting a Carnifex.
Notes from the field:
The vents on the back are almost always going to need to be greenstuffed. The fit is good, but even an expert assembly is going to leave a bit of a gap that detracts from the final product.
The (chickens have large) talons have damage built into them, but due to injection molding the dents tend to get filled in. Make sure you clear them out for authentic battle damage.
Some of these organic lines require more than just an exacto blade to clean up. Do yourself a favor and invest in a kit of different-shaped files (galeforce 9 makes an outstanding set). They not overly expensive, they work wonders, and they last a really long time. Do it.
Speaking of mold lines, they're so many on this model that you would do well to clean it multiple times. Even if you're OCD like me about your line cleanup, you're going to miss a couple the first time through.
There's a lot of big, flat surfaces on this model; you need to do something to compensate for this. I chose to texture my guys with line work, but I've seen all sorts of great solutions like gradients, modeled texture, and even freehand designs.
Remember to always paint the deepest part of the model first, that way you're much less likely to mess up an existing part of your work.
That's it for now, enjoy!