Would recommend as others have previously, to go for the siren first. Good expansion related drops and requiring little energy in slaying and ease of spawning from cows and pigs.
If you need mithril or elvenite, go for the royal siren and use fruit trees as others have said. The 500 shell tokens for the royal dark unicorn is hard to achieve if you have limited energy resources. Depending on how many shell tokens you can generate, you can leave till the last day to decide what to craft in the trade wagon as the crafts are only 15 min.
I used almost 10K energy and ended with all 3 sirens, 2 royal dark unicorns and 1 firefly. Will probably not craft anything else as the energy required to do so is too high vs item drops (would do another siren but it?s not craftable!). I guess this is why the royal dark unicorn does not have a max craft limit (at least not stated anywhere) like other energy giving craftable creatures do.