Whispike Survivors - Sword of the Necromancer

Whispikes have different stages. They start as a seed and then grow a little and then turn into a baby and then grow a few more times until they become an adult. You can wait for this to happen after playing so many games. It says next to the Whispike in the status screen this takes days, but don't worry, it's not real 24-hour days. It's usually just a few games for it to grow to different stages. They can get overripe which costs quite a bit more manure to save and they can also turn rotten which can't be saved.

You can accelerate this by clicking feed manure. Just give the seeds the amount of manure it asks for until it grows to the next stage and then the achievement is yours.