If you use an in-training item and then accept a challenge, the game treats it like a cup championship; you fight their monster three times instead of just once. When combined with all the XP you get from challenege matches, this becomes an absolutely broken way to gain XP very quickly.
Also items sell for their purchase price, not the shown sell price, and non-damaging attacks (slow, poison, etc.) hit you even when invisible.
 "Upgrade Complete 3"){kind=icon}
 "Monster Arena"){kind=icon}