How to make a chimera encounter harder?

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 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.