Player vs. Player battles are a core mechanic of Dark Summoner.
From the Battle screen, players can select an opponent and get a general view of their strength and party composition. However, there is no way to know their monster levels, making for some uncertainty when attacking.
In a successful PVP battle, the winner gains a small amount of XP, a portion of the player's gold (this amount has been estimated to be about 10% of the enem's gold, which caps when the enemy has below about 1000, in which case none is received), and a number of Battle Points related to how many BP that player has. In more recent versions, Honor Points are awarded instead.
Experience gained through PVP seems to be exp=2+Max(0, Enemy's level - your level). To acquire as much experience as possible, players tend to fight with the lowest possible BP formation with the highest strength.
Like, a single max level B+ could most likely beat five low level rank B monsters. Therefore if you are level 20 using a 12 BP max level monster, beating a level 30 player, you will gain 12 exp with only 12 battle points spent, therefore 1 minute -> 1 battle point recovered -> 1 exp gained with this setting. Of course you have to pick your opponent wisely.
There are quite some high level players leaving the game by gifting everything they have to the other players. If you are patient enough, you can keep refreshing the battle page until you found a high level player defended with only one single rank C monster. When you found him, attack him with any low battle point Rank B or above monsters. You can gain exp fast this way.
Rewards and Ranking
Player ranking by number of BP is tracked by the developers, and rewards are periodically given out based on ranking. The top players can receive valuable and rare items or Summon crystals, after which ranking is reset.
Guilds are ranked daily based on the total BP generated by Guild members. According to developers, bonuses are awarded based on these numbers, but specifics are unknown at this time.
'Note: 'that the BP rewards now have been changed to reward you with honor points which has the same function it should of had before.