My Japanese native language speaker in college had an obsession with the word "squirrel". We spent an entire year trying to teach her how to pronounce it.
You're not alone. "Squirrel" pops up as that one word most Germans seem to be insecure about even if they otherwise speak fluent English.
I think this is mostly because Germans are generally taught RP, where it is pronounced /ˈskwɪɹəl/ (skwe-rel). The American /ˈskwɝl̩/ (skwerl with a very faint r) is considerably easier to pronounce for German speakers.
Because neither the English /w/ nor the English /ɹ/ exist in German and because they're both pronounced in close proximity to each other, the combination tends to be confusing to pronounce (as with combinations of th and s/z).
I had trouble with the Japanese "ts" sound until I was taught that "ts" is the same sound as "pets" without the "pe.
Can you pronounce "squirt"? If yes, try pronouncing the "squir" portion of the word, dropping the "t". You should be pronouncing it like this reference, minus that hard "t" sound at the end [0].
Can you pronounce the rare English word "rill"? It's of Germanic origin so you should be able to. You should be pronouncing it like this reference [1]
If yes to both of the above questions, you can pronounce "squirrel".
Say "squir" from squirt, take a pause for a half second, then say "rill". Repeat the phrase faster and faster trying to shorten the pause between the two.
Alternatively, you can approximate the word "squirrel" from the English word "quarrel" by adding an "s" to the beginning. "s" + "quarrel". There will be a subtle mispronunciation with this method, but you should be able to pass in most conversation.
"Squirrel" is in fact a very difficult word to pronounce. Too many consonants squeezed into only two syllables -- sometimes even pronounced as a single syllable -- with a weird diphthong in between.
The ancient Greek version would have been much easier to express in Japanese: su-kyuu-ro-su.
Ssss-ka-wuh-rrr-llll