Whacko is a random enemy that can show up as early as depth 1190.
As a reference: I found him at depth 1225. So it took a while for him to show up in my game. Compared to other enemies at this point in game the Whacko is a bit more difficult (more health & stronger attack), but nothing to spectacular. Just keep improving your weapons with gems and you will be fine.
This is not a on-time encounter. He is added to the pool of random enemies that spawn as you move on and you will kill Whackos many more times.