I've found multiple sources instructing to use the つつ particle after the -masu stem of a verb only in formal or literary contexts. However, I sometimes find what appear to me to be contradictory examples.
Here's one from a source for JLPT training. It warns:
But later gives this example sentence of correct usage: ⑥ あんまり無理すると倒れるよ。少しは休みつつやりなよ。
This sentence appears to me to occur in a casual, spoken conversation. What gives? Is つつ for formal/literary use only, or not?