I've had a quick read through here too for you, and I would think it would be good to get a few more troll essence in order to meet the requirements of the current and one other in this part. You're right, trolls are more rare when the boars are released. You want to hold off building the swampy pond as they decrease dramatically once the pond is placed.
https://forums.storm8.com/showthread...=1#post1364393
You can then collect troll essence more leisurely as it drops from the troll buildings so you'll be able to replenish your stock soon enough.