I want to lower the CR for Pazuzu to 12. The idea is he is not yet fully on the prime plane and my players will be fighting him in this weakened state. Any ideas?
It is highly unlikely that he would go to the prime material plane at all. You don't become a demon prince by exposing a weakness. The reward for such a risk would have to be so great that it would be quite beyond a CR 12 adventure. Rather, he would send an avatar or emissary of some kind to act on his behalf - basically a less powerful version of himself but he won't die if it does.
If nothing like this exists, you can write one up by lowering his hit dice and eliminating some of his more powerful abilities. In lowering hit dice, he has fewer hit points, feats, skills points, and spell-like abilities. His AC, saves, DR, and ability DCs are all lower. If you want to keep a Pazuzu stand-in somewhat of the badass that Pazuzu himself obviously is, then you might still end up wiping out your adventurers. If you can't playtest the avatar, at least compare it to something else that already exists, and compare it to your partys' mechanical abilities. Are their attack bonuses high enough to score hits? Can they succeed on saving throws? Can he fail saving throws against them? Can he miss attacks? Etc...
Being a avatar type replicate of himself will be what I go for. On another thread someone suggested the accursed template and I think I'll use that for the stats. Thanks for the input.