Personally, when I did mine, I painted Merry and Pippin individually, however I chose not to attached them with superglue or greenstuff, and simply used a small piece of BlueTac to hold them in place.
This meant I could remove them from Treebeard without causing any damage to him.
To paint Merry and Pippin, I drilled a hole into their butts, and used a wooden cocktail stick on each, which gave me something to hold when painting them.
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I ask because I have not yet painted a model of this scale and difficulty yet
I have always found, the bigger the miniature, the easier it is to paint. Treebeard is one of the easiest.
This article may help, although I personally would do the eyes yellow and not red..
Click HereMy treebeard....