"Hey sis..."
"Yeah bro?"
"...Is it just me, or are those monsters we froze familiar?"
"What do you mean?"
"Like, I could have sworn I've seen them in those really old movies."
"Oh, they're just universal silly! They've been in everything!"
"Am I thinking too hard again?"
"I think so!"
Popo and Nana then stopped in their tracks to see a flea man howling as he jumped at them, only for the two to freeze him in a block as they then smashed it to pieces with their wooden mallets, continuing their way through the haunted castle as Nana reassured Popo through their trek.
And as to how they dealt with the Grim Reaper before? Well it's simple! They just froze him up and walloped on him with their hammers!