Skilling up at level 70.

This could go for any monster at their cap but do you find it better to try and skill up to 20 before max leveling the card, vice versa, or something in between?

Leveling up skills early is certainly cheaper.

If you have a complete team and can handle waiting for all of the skills to level then i would say to skill it up first.

If you are working on your first team then prioritize how you see fit, but for a first team i find the levels more important.

Just my opinion.

Skilling up used to have a high success rate at low exp levels, but they fixed that a while ago. So it doesn't really matter now... skill up when you can, and level up when you can.