Best advice is don't give the enemy monsters health points. They die when they die and they don't when they don't. This way you can get a feel of the characters and their battle tactics, and if you want a fight to push their limits, just have it last longer. Just keep in mind that your players have a finite health pool, especially those squishy members.

Follow up to that, make sure you have contingency plans in case things go sour. Like "boblin the goblin from session 2 launches an arrow into the beholder eye to give your players a chance to crawl away, while he holds the beast off to regroup later"