What specifically is the struggle with core? Front/back? Maintaining shape for a skill/drill, difficulty performing a specific skill/drill, general fatigue, routine fatigue, posture breakdown? Lots of possible struggles. If you have something specific in mind, yell back and perhaps the forum's sage's can provide some insight.
For me, generally speaking, our USA G L6-10's struggle with keeping a "smart butt" (tight gluteal muscles) and strong lower abs. They have been the hardest targets for our athletes. Some good core training/ injury rehab vids can be found at Larry Nassar's Youtube channel (Gymnast Doctor -
http://www.youtube.com/user/gymnasticsdoctor?feature=watch ). Check it out and mine the good vids for ideas!
Best, SBG -