Advice for running a challenging chimera fight?

[pawsplay]

You could use the more deadly Rock Chimera variant:

The chimera prefers to surprise prey. It often swoops down from the sky or lies concealed until it charges. The dragon head can loose a breath weapon instead of biting. Several chimeras attack in concert.

 

[frankthedm]

Add a medium viper (bite +12 [1d4+4, poison 1d6 /1d6 Con, Fort DC = breath]) as it's tail.

A single monster rarely works for a good challenge, 1d3+2 Chimera would be aa better fight.

if you really want a lone chimera, let it attack will all of it's heads ala a hydra and have the non lion-heads be sunderable.

Advancement to huge might make it a tougher fight.

 

[pawsplay]

I can tell you from experience that a fully advanced, Huge chimera with a 2nd level fighter lizardfolk using it as a mount is a lot tougher than its EL suggests.

 

[phindar]

If you apply the Chimeric Template (MM2) to a lion (which is what the chimera is supposed to be anyway), you end up with a chimera with Imp Grab, Rake and Pounce. If you give it Flyby Attack, it can has the option of either charging for the pounce or swooping by and Imp Grabbing an single opponent before it continues on it's move.

Chimera are INT 4, so slightly above animal intelligence. They also can speak Draconic, which makes it easier for them to direct lackeys that can speak or understand that language, such as kobolds. (And kobolds are smarter than chimera, so even if the chimera is the boss by virtue of sheer power, having devious trapmaking underlings can help out. Perhaps they used to work for the dragon.)

Another vote for difficult terrain, including slush, quickslush, icy terrain, ravines and so on. Anything from Frostburn would fit in well here. If the lair is in the tundra, the environment itself can be challenging. Particularly since the chimera can fly above it. Chimera have poor maneuverability, but they can also Hover, so even in tight quarters they can stay airborne. Also, hovering in an area of loose debris (like snow and slush) creates a cloud that gives concealment. Since it's breath weapon is an AoE, it won't be subject to the miss chance. (Granted, it's only 20%, but its essentially free anytime the chimera hovers within 20' of the ground.)

 

[frankthedm]

And since chimera minis are kinda hard to come by, and you probably won't find them in medium or huge unless you make them yourself, here is a quick counter sheet.

http://img183.imageshack.us/img183/6919/chimerafamilygk1.png
img183.imageshack.us/img183/6919/chimerafamilygk1.png