I would store the unit and the battle-location in a seperate table.
this table would also include fields for when the monster was first attacked, by which player and by which guild.
Now every player of tht guild that visits the same location get's a to chose to either find and attack a new monster or attack an already found monster.
But the monster will only be fightable 5 minutes (or so) from the last attack. So the guild would have to work together and plan it accordingly to actually kill it.
And when the monster is killed everyone that fought it will gain some xp, in relation of how often he fought the monster. (So a player that attacks 3 times and a player that only attacks once will get 3/4 and 1/4 of the total experience)
And yes, this will probably only be done for boss monsters, quest related monster or really, really strong monster.