Here is the full list of all 20 99 Waves achievements.
Slay the King Slime.
Defeat the Iron Knight.
Defeat the Cyborg Warper.
Win Round #2 against Treasure Goblin.
Eliminate the Aggregate of Grudges.
Defeat the God.
Receive kindness from a neighbor woman.
Have the gravekeeper dig a magical portal.
Play Endless Mode.
Talk to the merchant.
You are a true legend.
Thank You for Playing!!
Start in Accel Mode.
You can now play as Shadow Ninja.
Maybe next time!
Hit the jackpot on the slot machine.
Die 5 times.
Die 10 times.
Die 50 times.
Die 99 times.