Tips for pronunciating tone value "21"?
It is possible to analyse English intonation with Chinese tonal contours, a favourite topic of mine that tends to upset a lot of people who believe their language is not tonal. This remains controversial, but not so much for those working in voice recognition technologies who actually look at the data.

In this response I refer to the surface phonetic tone in square brackets [] as is customary in IPA (which I assume you're referring to but not explicitly stated in your question), and the underlying phonemic tone number (as in 平上去入) like this //.

The tonal contour [21] would appear in this English sentence on the word "for":

"What are you looking for?"

Just isolate that word "for" and repeat it over and over and feel the [21] on it. To a certain extent, this would be considered either /3rd/ or neutral tone in Mandarin, although neutral can manifest at different heights.

In the same sentence, we can also make "you" a [21] tone also, but you'd need to change the overall intonation:

"What are you looking for?!"

I would say [21] gets spread over "are you".

Both Cantonese and the Hokkien (Southern Min) variety I speak have [21], which is called a third tone in Hokkien, and I believe a /4th/ tone in Cantonese, although I think it starts a bit higher.

As for the rest of the sentence, I would label "what" as mid-rising [34], getting stretched out over four syllables "what are you look", and [2] on "-ing".

What3 are3 you3 look4 -ing2 for21?
*What*53 are2 you1 look4 -ing2 for21?!

Another tough tone would be [11] and you may feel like you'll get that confused with the [21] above. So I'll try to help. You can find this appearing on "what" in the following English sentence if you apply extra stress to the first word and say the sentence like a response to somebody else and not in an exclamatory way:

That's what I'd like to do.

Actually the [11] contour maintains through "what I'd like" with a slight rise of [23] on "do". I would say the word "that's" has a high rising [45] contour. Practise this first in English and try to isolate the words exactly as you normally say them and try to isolate the tone you're using. This will help you map between what you can naturally do in English (although depends on the variety of English you speak) and how to produce the tones right in Chinese every time. Here's my analysis of that sentence:

That's45 what1 I'd1 like1 to1 do23.

An alternative intonation for comparison:

That's34 what3 I'd3 like3 to4 do53!

(I'd prefer to put those numbers into ruby text, but I don't think the HTML here supports it).

This [11] appears in the Sixian Hakka variant I speak on the /2nd/ tone and in /1st/ tone sandhi followed by /2nd/ tone. I believe it also appears in Cantonese /6th/ tone (non-入聲 if you don't count to /9/), although that tone may have some dipping perhaps more akin to the [21] you mentioned above, I'll have to check my sources.

Good luck with training them. It takes time and patience. The better you get at this tone training, the more luck you'll have with many more languages, whether tonal or not. You'll hear so much more that's going on that most people were never aware even existed.