|Douglas Muir 406|
If you're a Wiz 10 / Diabolist 2, you can use Planar Binding to call up all sorts of nasties to fight for you. And, really, you should be doing that. (Subject to your DM agreeing, of course.) If you have an Osyluth or a pack of Nessian Hell Hounds working for you, then you don't really need the imp to enter combat. Give it UMD and use it as a wand platform.
-- The Nessian Hell Hound has no SR, a lowish Will save, and a laughable 6 Cha. So if you're looking for some straightforward combat muscle, call up a couple of these puppies. They can't fly or teleport, and they don't have any SLAs -- all they do is bite, trip, and breathe fire. But they're great bodyguards and, unlike more intelligent outsiders, are unlikely to plot horrible revenge against you. You can call up more powerful creatures, but try a couple of these guys first and see how it works out.
Have you read DMDM's Guide to the Diabolist?