Used properly, your pit can be a far more valuable training aid for your creatures than your training room. Training creatures in your pit does not cost you any gold and has the added benefit of providing entertainment for your other creatures. Here are a couple of tips for using your pit effectively:

• Don't let your creatures dies in the pit; it defeats the purpose of training them. Drop a few imps into the pit to drag your fallen creatures to safety. Better yet, if you have some time to monitor the pit, use the heal spell to restore the health of injured creatures. This will allow them to fight longer and advance farther in levels.
• If you are low on many and imps are in short supply, pick up injured creatures yourself and drop them outside of the pit. After they regenerate you can drop them back in the pit for some more training.
• Try to place similar creatures in the pit. Since they are evenly matched, you'll get longer fights - and more training before needing to intervene to heal.
• If you are busy with other parts of your dungeon, drop a few creatures with ranged attacks in your pit. They will fight each other for a longer amount of time before requiring your intervention.