Kuriotates suggestions (and one Lumbermill suggestion)

[MagisterCultuum]

You need OO's slavery civic to capture the elves that will build your Leaves empire latter in the game. Sacrifice the Weak is the AV civic, not the OO one. That was probably just a typo on your part.

 

[Chip56]

You need OO's slavery civic to capture the elves that will build your Leaves empire latter in the game

While this helps, its not necassary.
If you can capture elven workers you get them even without slavery.
But its sure easier if you have 25% chance to get a slave from a military unit