I wrote down a list of the "best" places to farm these mobs, however did not take into consideration level sync.
As for Treants/Saplings you can farm in Caedarva Mire, Dark, Lightning and Water Weather. In Woodlands or Thuckets, only Wind Weather is available, and not that frequently.
You can also find a Sync, level 15-~20 is suffucuent and kill saplings in the Canyon, These die a lot faster then the higher ones, and there is wind and earth weather there fairly frequently.
As far as spiders, we need Lightning, Wind, Fire or Light weather. So far as i am aware there are no sliders that roam in an Area with Light weather (barring Nyzul) so the Options are as follows:
Boyhada Tree when Lightning Weather is out, but as said, Water is the more common element there.
Thickets or Woodlands with Wind weather, the only weather available there, however, this is also not too common.
Kuftal Tunnel has both Fire and Wind weather, which is possibly the best bet.
The final Place is Quicksand caves, but Earth weather is Very common there, as opposed to fire, however its a possibility.
As far as the next step, its limited to Light weather or day, so picking the day is the best bet here. I plan to ride that and the following step (arcana) out in RoMaeve, while doing FoV for some Quite good exp. Wether its Lightsday or not, 1300 exp /page + normal killing will not be bad exp.
The final stap is 150 Beast type mobs, killed with Banish/Holy. For this im poanning on going to U Range. There are a variety of Beast type mobs here, all yielding exp, and not too hard to fight.
Thats what my path looks like, the treants and spiders will be the hardest part, as they are weather mobs. The Lightday ones, will take a while, but at least you can be certain of when a Lightsday is, so i think it will be easier.
Good Luck :D
Chocobo Status: Retired Green, Black, Red, Yellow
Whm/Blm/Rdm/Mnk/Thf/Pld/Blu/Smn/Dnc/Pup/Bst/Drg 99, War 91, Brd/Sch/Sam/Nin/Cor/Drk/Rng/Geo/Run 60
Alch 72+1, Cook 66, CC 61+1, Wood 61, Synergy 56, Bone 54, Leather 51, Gold 49, Smithing 45, Fish 15 http://ffxi.allakhazam.com/profile.xml?143946