Tip 1: Always take missions that give you a high ratio of experience gained vs CR spent. For example, if there is a 2EG:2CR mission and a 3EG:2CR mission, of course take the one that gives you more experience for the same CR.
Tip 2: Going up a level will refill your CR metre, so try to time your metre running out just as you make it to the next level. For example, it would be very inefficient to get 100/100 experience to the next level when you're at 28/30 CR because you'll only get 2 free CR. What you could have done was the last time you emptied the CR metre (0/30) if you were at 98/100 for experience, make sure to come back online really soon when you can get that last push to get you to 100/100 experience, which will then give you 30/30 CR with 30 free CR.
Tip 3: It's all about the medals!! Not just doing what it takes to earn them, but specifically reading what you need to get each one and then doing it. For example, if a medal only requires you to own X number of things in your inventory, go out and buy lots of cheap stuff.
Combine those tips and you'll end up riding a wave of momentum going from level to CR to medal to level to CR to medal, etc.