Personally I would say that you shouldn't have arenas that give away extra EXP/happiness. That one is obviously designed for power-leveling a pokémon by giving it at least one additional lucky egg boost, for example. While the level of EXP it grants is at least possible for the pokémon to earn in that battle (if you nix the ditto's send-out EXP, that is; as written you get more), it makes it much easier to send all the points where you want them. I don't think that having to evaluate these kinds of arenas is something you want to have to deal with, because people will definitely try to slip stuff past; I would just ban them outright.