While doing leves you just need to active them and run to the yellow area that is marked on your map. Once there your minimap will display red dots which are the leve targets. There are always enough mobs for everyone but if there are a lot of people you may get lag while trying to attack them and repeatedly get an error that says "Target is already engaged."
Edit: What they meant by instanced was that you can only attack those certain mobs while on that leve. Also the game spawns a set of mobs for every person currently on the leve.
Edited, Sep 25th 2010 4:38pm by sima2
Final Fantasy XIV
Character: Ryiah Nustar--Dunesfolk Lalafell♀