I'm looking for tips for creatures or obstacles that could spice up an encounter.
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?