I'm looking for tips for creatures or obstacles that could spice up an encounter.
Background:
The battle's purpose is to be an entrance test for the organization the campaign is centered around. The party consists of 5 fifth level characters (with multiple healers), and I was thinking that the battle could be centered around a Chimera. But not only would it be boring with only a Chimera it would also be too easy (the battle takes place in a colosseum/arena type environment located on the outskirts of a big city).
Are there any instructions in the published books to make interesting additions to combat encounters, such as a) making the combat fit a specific location thematically, b) mixing different kinds of monsters, or c) adding traps and other interactive things to the battleground?
