By Tim Hanlon BARCELONA, Dec 13 (Reuters) - Leaders Barcelona need to reproduce the free-flowing tactics that allowed them to crush Celtic in the Champions League when they face a tricky La Liga outing against Villarreal this weekend, coach Gerardo Martino said. The Catalan side shrugged off a recent dip in form, during which they were beaten by Ajax and Athletic Bilbao, with an inspired display to beat Celtic 6-1 on Wednesday. After securing top spot in their Champions League group, they will be keen to preserve their number one status in La Liga, where they lead Atletico Madrid on goal difference. More...