Core strength is not climbing specific (I assume): I usually use some energetic song for timing and then do various exercises of which I don't know the English names sorry. Like holding the extended push up position and then touching the elbow and knee of opposite sides repeatedly. Or similar position on one arm and the side of your foots, body sideways, then lift arms and legs or move hip up and down. Then some exercises for abs and back.
In the end I use like 4 main exercise with variations. If I needed to make them harder (never got to that point), you can do the same on balance boards a…