"I have seen an elephant once."
Tłumaczenie:Widziałem raz słonia.
'widzieć' is a non-perfective verb, hence this should instead be translated as 'kiedyś zobaczyłem słonia' (with the perfective verb 'zobaczyć'). Or what do I miss here?
You miss the fact that if you talk about your experience, you should use an imperfective verb (widzieć). If you talked about one specific event when you saw an elephant, then you should use "zobaczyć".
Uhm.. is it possible to differentiate between these in English? How would the 'zobaczyć' version be translated?
Mihxal, Yes, he had seen an elephant once. That qualifies as your, "one specific event", doesn't it?
It may refer to the fact that you have already seen an elephant, that is it isn't new to you but without specifing when it happened and in what circumstances. In other words, you have already experianced it but it doesn't say when it happened.