While there's a concern with many routes of the claiming system, I think to counter the primary power leveling aspect would be, "XP is awarded in proportion to the damage the party contributes to defeating the enemy." and loot goes toward "who tagged it first".

Cons: Griefing. You can continue to attack someone else's monsters and reduce their XP.
Debate based on ideals: Prevents the current power leveling where a weaker party can claim a group of monsters to be destroyed by higher level players.
Pro: A lower level player can still obtain XP when a higher level player is rounding up a large host of lower level enemies for farming purposes. In the current system, the lower level player cannot benefit when a higher level player does this.
Pro: Multiple parties working together in strongholds or other public locations can share in the XP rewards and SB growth, i.e. a linkshell of 12~24 players invading a stronghold.