As @ChristophIV has mentioned, you need to get 15 attacks in a row before the enemy team takes their first turn. The feat is not to take 15 attacks in a row at any time during the battle..
@furyousgeorge74 In English, "before the enemy takes a turn" is generally taken to mean before their first turn. E.G., "The enemy decimated me before I could take a turn."
I think the problem people have with interpreting this particular feat is that there is a similar feat for GCs that will count "15 turns in a row" which allows for being in between enemy turns.
If you have to say "generally", then it is not clear. Since people have to invest time and in-game currency into these feats, spending the extra minute or two to write precise descriptions is only fair. CG can afford to pay their content writers well. They should be precise, especially since English is the source language from which all the translations are done.
