She had two choices, try to dodge it, which wouldn't be easy or try to block it - which would definitely hurt. Selina decided to take the blow, the impact of Dinah's leg knocking her back. Selina let out an audible 'oof' at the power behind it, stumbling slightly but finding her balance quickly - clearly she was lining her up for something else and all Selina could think was to try and get her down - but still hurting from the hit she took a second, a second to long to decide her next move...