__ Stroke, Most Tiring of All Swimming Strokes

Puzzle 12 of Same Letter at Both Ends Easy Pack

Question is: __ Stroke, Most Tiring of All Swimming Strokes

and the answer is: BUTTERFLY

Word Meaning:

BUTTERFLY — A flying insect of the order Lepidoptera, distinguished from moths by their diurnal activity and generally brighter colouring.

Definitions from Wiktionary.