If you have another interpretation to Krillin's quote then what is it? Why is he using Freeza of all people to hype up Cell?
I don't think it's totally power-related (though it is somewhat power-related, in a way, more on that later). It's more character-related, though that's a bit of a poor way to word it. I think it's more related to Cell's intentions, like Clearin has already said. #17 and #18 are stronger than Freeza and more threatening from purely a power perspective for instance, but they didn't even want to do anything besides have fun and kill one major character.
Think about it. Just the statement on its own. "Terrible" is often a way to describe huge power, but it seems pointless to ask "are you ready to take someone stronger than Freeza?" knowing that Goku had quite literally planned out the last 3 years for exactly that, and yet Kuririn does ask that, so I don't believe it's as concrete as "are you ready to take someone stronger than Freeza?" It adds even less - if you interpret from a pure power sense - when Piccolo had already asked a more effective version of the question earlier: "You ready for these androids?", "I dunno, I'll know when I get there." It already goes without saying that he is excited and afraid of someone stronger than Freeza. Actually, I take that back - it goes
with saying, before Goku even trained:
http://i.imgur.com/Bxp2AML.jpg
Not everything has to lead to a strict power chain.
I've given my reason why he doesn't mean Freeza > 17/18. I'll say it again just in case. Cell can potentially take out the andoids, so Cell is their new biggest threat, he can be grouped with the androids. Since 17 and 18 are acknowledged in the conversation we know Kuririn isn't saying Freeza > 17 and 18. That's the conclusion you'd come to if you don't pay attention to the context. So what's your interpretation? Freeza is still being used in the context of power within a dire situation. Not 19 or 20. So why should we assume 19 and 20 are stronger?
I'm not saying to assume 19/20 are stronger without evidence, I'm just saying that the line doesn't necessarily mean they are. In fact, if this line was the only piece of evidence about their power, then I would agree with you because then there's just no other option.
Like I said above, I don't see the statement as much of a power statement. It's kind of a power statement, but only in the sense that beating Cell will be challenging to the current set of Z fighters, much like how beating Freeza was a challenge to the past set of Z fighters on Namek, if that makes sense. Otherwise, it's mostly just saying that Cell is an awful villain of comparable status to Freeza, or worse than him, rather. Otherwise it's just a wonky, pointless statement that feels out of place to me.
