It's okay to just let Frederick one-shot the Mages and Myrmidons. Don't know what it is like on Hard, but on Lunatic, he can't one-shot the Barbarians. Depending on how you play your cards, you can get up to four levels total between Robin and Chrom. Chrom should have no problems with reaching Lv 3 (4 if you are either very lucky or very clever), where as if you have a good feeling about Chrom, you can start to feed Robin nice and early and get her a level or two.
It's really sensitive, but I find it's best if you level up Chrom so he is strong as he can be for Ch1 and 2, and immediately start feeding Robin in Ch3, and let her have any scraps you can afford for Ch2. Robin should be strong enough to hold her ground in Paralogue 1 and Ch4. After this point, she'll start to pick up an absurd amount of momentum.