Psykeepers have all the right skills to challenge and destroy anyone but the Rangers. Any short range attacks on Psykeepers will be useless against them. So Blades, Knights, Ringmasters, and Billposters are pretty much useless and defenseless.
Because all your skills are magic if you go up against a Ranger you'll be defeated easily. If you get hit by Silent Shot, all your skills are rendered useless for 15 seconds. By the time you're able to cast a spell against them, you'll probably already dead. Against monsters, you're utlimately be the champ and will barely get touch. You have skills that will block and stop monsters in their tracks.
Giant hunting with a Psykeeper will guarentee a victory almost everytime because of skills which will make monsters immovable. While you're attacks are high powered but slow casting, killing off gaints will take longer than most other melee classes. However you'll never be hit...