The exp calculations are still a bit bugged. Here's an example as of Feb 26, 10am server time. The Sage Cats are lvl 66. My mule is lvl 53. My mule does not receive any exp when the bishop kills. (Sage Cats give 816 exp per kill without HS.) 1. Both bishop and mule are in a party. Bishop receives 718 exp per kill. (wrong) 2. Bishop is not in a party. The bishop gets 897 exp per kill. (correct) 3. Bishop is in a party alone. Bishop receives 718 exp per kill. (wrong) Hope this helps. Until there's a fix, don't train while in a party alone. If you're solo training with a mule (HB,SI,SE...) that is too low level to receive exp, you should kick the mule after buffing.
