Finally, I beat Redwoodtail
Here's the strat I used:
I used the Diamond prefix for the diamond scrap. Then I trained to get my base health to at least 600 so Redwood wouldn't one-shot me. Then on my first turn, I used an egg to get her attack down. Then I used the thrasher beak to up my attack. Then I just did one normal attack and took her down.
There is a bit of RNG, so you might have to keep trying until you get lucky