I've never had a problem with event girls.

But with Bremen and Grunt and Donatien and Ninja Spy I'm missing 2/1/2/2 respectively. Since I've been working the contests pretty hard I know it might take me months to get all available girls, but I'm usually #1 in the contests. I can't spend energy since I've completed the final level available and I can't give girls XP since I can't earn new girls so I usually loose those ones but stuff like Mind and Body Synergy or Show off your Skills at the Casino are easy to do.