How to make a chimera encounter harder?

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 organisation the campaign is centered around. The party consists of 5 fifth lvl 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 to easy so (the battle takes place in a Colosseum/arena type environment located on the outskirts of a big city).

So yeah if you have any tips on creatures or obstacles/ traps that could fit I would greatly appreciate help.