Oh boy. Okay.
So basically, Kecleon only appears as a shopkeeper inside dungeons. You can buy and sell him stuff if you have enough money on you. However, you probably won’t, since taking money with you into dungeons is super dangerous.
So instead, you can steal from him. But doing this will continuously spawn super overpowered Kecleons that move multiple times in one turn. Even defeating one is a major feat, and they WILL gang up on you. So the usual course of action is to try and survive as you rush to the exit.
That nosepass… is taking out multiple of them in one move.
Kecleon is DESIGNED to be neigh-unbeatable in PMD. I think I remember reading that EVERY single one of their stats (excepting HP, I think?) is 255 (which is the highest possible in the first 2 PMD games). The fact that someone was able to get an attack off that did over 300 damage to each Kecleon around them is nothing short of god-like.