Rules
When doing playtesting, you should be testing a creature or challenge against a group of five relatively balanced characters. If the encounter's PAR is 8, then all five members of the party should be level 8. If the PAR is <1, then a single PAR <1 creature should be easy or non-challenging to the party.
What you're looking for is fun. If it's PAR 8, was the encounter fun and a bit challenging to your group of 8th level characters? If not, was it too easy? Was it boring? Or was it too difficult? This is the info we are looking for.
Choose a fairly neutral setting for the challenge to occur. If you are fighting a 50-foot tall bird, don't start the fight in a 20-foot room. Likewise, don't face an electric-powered automaton in a lightning storm (unless that automaton would *only* appear in a lightning storm). Make the environment natural to the fight, and start the adventurers a logical ways away (between 10 and 50 feet, depending on the battlefield).